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<div class="section" id="plac-parsing-the-command-line-the-easy-way">
<h1><a class="toc-backref" href="#id15">Plac: Parsing the Command Line the Easy Way</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Author:</th><td class="field-body">Michele Simionato</td>
</tr>
<tr class="field"><th class="field-name">E-mail:</th><td class="field-body"><a class="reference external" href="mailto:michele.simionato&#64;gmail.com">michele.simionato&#64;gmail.com</a></td>
</tr>
<tr class="field"><th class="field-name">Date:</th><td class="field-body">May 2012</td>
</tr>
<tr class="field"><th class="field-name">Download page:</th><td class="field-body"><a class="reference external" href="http://pypi.python.org/pypi/plac">http://pypi.python.org/pypi/plac</a></td>
</tr>
<tr class="field"><th class="field-name">Project page:</th><td class="field-body"><a class="reference external" href="http://plac.googlecode.com/hg/doc/plac.html">http://plac.googlecode.com/hg/doc/plac.html</a></td>
</tr>
<tr class="field"><th class="field-name">Requires:</th><td class="field-body">Python 2.3+</td>
</tr>
<tr class="field"><th class="field-name">Installation:</th><td class="field-body"><tt class="docutils literal">easy_install <span class="pre">-U</span> plac</tt></td>
</tr>
<tr class="field"><th class="field-name">License:</th><td class="field-body">BSD license</td>
</tr>
</tbody>
</table>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#plac-parsing-the-command-line-the-easy-way" id="id15">Plac: Parsing the Command Line the Easy Way</a><ul>
<li><a class="reference internal" href="#the-importance-of-scaling-down" id="id16">The importance of scaling down</a></li>
<li><a class="reference internal" href="#scripts-with-required-arguments" id="id17">Scripts with required arguments</a></li>
<li><a class="reference internal" href="#scripts-with-default-arguments" id="id18">Scripts with default arguments</a></li>
<li><a class="reference internal" href="#scripts-with-options-and-smart-options" id="id19">Scripts with options (and smart options)</a></li>
<li><a class="reference internal" href="#scripts-with-flags" id="id20">Scripts with flags</a></li>
<li><a class="reference internal" href="#plac-for-python-2-x-users" id="id21">plac for Python 2.X users</a></li>
<li><a class="reference internal" href="#more-features" id="id22">More features</a></li>
<li><a class="reference internal" href="#a-realistic-example" id="id23">A realistic example</a></li>
<li><a class="reference internal" href="#keyword-arguments" id="id24">Keyword arguments</a></li>
<li><a class="reference internal" href="#plac-vs-argparse" id="id25">plac vs argparse</a></li>
<li><a class="reference internal" href="#final-example-a-shelve-interface" id="id26">Final example: a shelve interface</a></li>
<li><a class="reference internal" href="#plac-vs-the-rest-of-the-world" id="id27">plac vs the rest of the world</a></li>
<li><a class="reference internal" href="#the-future" id="id28">The future</a></li>
<li><a class="reference internal" href="#trivia-the-story-behind-the-name" id="id29">Trivia: the story behind the name</a></li>
</ul>
</li>
<li><a class="reference internal" href="#advanced-usages-of-plac" id="id30">Advanced usages of plac</a><ul>
<li><a class="reference internal" href="#introduction" id="id31">Introduction</a></li>
<li><a class="reference internal" href="#from-scripts-to-interactive-applications" id="id32">From scripts to interactive applications</a></li>
<li><a class="reference internal" href="#testing-a-plac-application" id="id33">Testing a plac application</a></li>
<li><a class="reference internal" href="#plac-easy-tests" id="id34">Plac easy tests</a></li>
<li><a class="reference internal" href="#plac-batch-scripts" id="id35">Plac batch scripts</a></li>
<li><a class="reference internal" href="#implementing-subcommands" id="id36">Implementing subcommands</a></li>
<li><a class="reference internal" href="#plac-interpreter-call" id="id37">plac.Interpreter.call</a></li>
<li><a class="reference internal" href="#readline-support" id="id38">Readline support</a></li>
<li><a class="reference internal" href="#the-plac-runner" id="id39">The plac runner</a></li>
<li><a class="reference internal" href="#a-non-class-based-example" id="id40">A non class-based example</a></li>
<li><a class="reference internal" href="#writing-your-own-plac-runner" id="id41">Writing your own plac runner</a></li>
<li><a class="reference internal" href="#long-running-commands" id="id42">Long running commands</a></li>
<li><a class="reference internal" href="#threaded-commands" id="id43">Threaded commands</a></li>
<li><a class="reference internal" href="#running-commands-as-external-processes" id="id44">Running commands as external processes</a></li>
<li><a class="reference internal" href="#managing-the-output-of-concurrent-commands" id="id45">Managing the output of concurrent commands</a></li>
</ul>
</li>
<li><a class="reference internal" href="#experimental-features" id="id46">Experimental features</a><ul>
<li><a class="reference internal" href="#parallel-computing-with-plac" id="id47">Parallel computing with plac</a></li>
<li><a class="reference internal" href="#monitor-support" id="id48">Monitor support</a></li>
<li><a class="reference internal" href="#the-plac-server" id="id49">The plac server</a></li>
<li><a class="reference internal" href="#summary" id="id50">Summary</a></li>
<li><a class="reference internal" href="#appendix-custom-annotation-objects" id="id51">Appendix: custom annotation objects</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="the-importance-of-scaling-down">
<h2><a class="toc-backref" href="#id16">The importance of scaling down</a></h2>
<p>There is no want of command-line arguments parsers in the Python
world. The standard library alone contains three different modules:
<a class="reference external" href="http://docs.python.org/library/getopt.html">getopt</a> (from the stone age),
<a class="reference external" href="http://docs.python.org/library/optparse.html">optparse</a> (from Python 2.3) and <a class="reference external" href="http://argparse.googlecode.com">argparse</a> (from Python 2.7).  All of
them are quite powerful and especially <a class="reference external" href="http://argparse.googlecode.com">argparse</a> is an industrial
strength solution; unfortunately, all of them feature a non-negligible learning
curve and a certain verbosity. They do not scale down well enough, at
least in my opinion.</p>
<p>It should not be necessary to stress the importance of <a class="reference external" href="http://www.welton.it/articles/scalable_systems">scaling down</a>;
nevertheless, a lot of people are obsessed with features and concerned with
the possibility of scaling up, forgetting the equally important
issue of scaling down. This is an old meme in
the computing world: programs should address the common cases simply and
simple things should be kept simple, while at the same time keeping
difficult things possible. <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> adhere as much as possible to this
philosophy and it is designed to handle well the simple cases, while
retaining the ability to handle complex cases by relying on the
underlying power of <a class="reference external" href="http://argparse.googlecode.com">argparse</a>.</p>
<p>Technically <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is just a simple wrapper over <a class="reference external" href="http://argparse.googlecode.com">argparse</a> which hides
most of its complexity by using a declarative interface: the argument
parser is inferred rather than written down by imperatively.  Still, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is
surprisingly scalable upwards, even without using the underlying
<a class="reference external" href="http://argparse.googlecode.com">argparse</a>. I have been using Python for 9 years and in my experience
it is extremely unlikely that you will ever need to go beyond the
features provided by the declarative interface of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>: they should
be more than enough for 99.9% of the use cases.</p>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is targetting especially unsophisticated users,
programmers, sys-admins, scientists and in general people writing
throw-away scripts for themselves, choosing the command-line
interface because it is quick and simple. Such users are not
interested in features, they are interested in a small learning curve:
they just want to be able to write a simple command line tool from a
simple specification, not to build a command-line parser by
hand. Unfortunately, the modules in the standard library forces them
to go the hard way. They are designed to implement power user tools
and they have a non-trivial learning curve. On the contrary, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>
is designed to be simple to use and extremely concise, as the examples
below will show.</p>
</div>
<div class="section" id="scripts-with-required-arguments">
<h2><a class="toc-backref" href="#id17">Scripts with required arguments</a></h2>
<p>Let me start with the simplest possible thing: a script that takes a
single argument and does something to it.  It cannot get simpler
than that, unless you consider the case of a script without command-line
arguments, where there is nothing to parse. Still, it is a use
case <em>extremely common</em>: I need to write scripts like that nearly
every day, I wrote hundreds of them in the last few years and I have
never been happy. Here is a typical example of code I have been
writing by hand for years:</p>
<pre class="literal-block">
# example1.py
def main(dsn):
    &quot;Do something with the database&quot;
    print(&quot;ok&quot;)

if __name__ == '__main__':
    import sys
    n = len(sys.argv[1:])
    if n == 0:
        sys.exit('usage: python %s dsn' % sys.argv[0])
    elif n == 1:
        main(sys.argv[1])
    else:
        sys.exit('Unrecognized arguments: %s' % ' '.join(sys.argv[2:]))

</pre>
<p>As you see the whole <tt class="docutils literal">if __name__ == '__main__'</tt> block (nine lines)
is essentially boilerplate that should not exist.  Actually I think
the language should recognize the main function and pass the
command-line arguments automatically; unfortunaly this is unlikely to
happen. I have been writing boilerplate like this in hundreds of
scripts for years, and every time I <em>hate</em> it. The purpose of using a
scripting language is convenience and trivial things should be
trivial. Unfortunately the standard library does not help for this
incredibly common use case. Using <a class="reference external" href="http://docs.python.org/library/getopt.html">getopt</a> and <a class="reference external" href="http://docs.python.org/library/optparse.html">optparse</a> does not help,
since they are intended to manage options and not positional
arguments; the <a class="reference external" href="http://argparse.googlecode.com">argparse</a> module helps a bit and it is able to reduce
the boilerplate from nine lines to six lines:</p>
<pre class="literal-block">
# example2.py
def main(dsn):
    &quot;Do something on the database&quot;
    print(dsn)
    # ...

if __name__ == '__main__':
    import argparse
    p = argparse.ArgumentParser()
    p.add_argument('dsn')
    arg = p.parse_args()
    main(arg.dsn)

</pre>
<p>However, it just feels too complex to instantiate a class and to
define a parser by hand for such a trivial task.</p>
<p>The <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> module is designed to manage well such use cases, and it is able
to reduce the original nine lines of boiler plate to two lines. With the
<a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> module all you need to write is</p>
<pre class="literal-block">
# example3.py
def main(dsn):
    &quot;Do something with the database&quot;
    print(dsn)
    # ...
 
if __name__ == '__main__':
    import plac; plac.call(main)

</pre>
<p>The <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> module provides for free (actually the work is done by the
underlying <a class="reference external" href="http://argparse.googlecode.com">argparse</a> module) a nice usage message:</p>
<pre class="literal-block">
$ python example3.py -h
</pre>
<pre class="literal-block">
usage: example3.py [-h] dsn

Do something with the database

positional arguments:
  dsn

optional arguments:
  -h, --help  show this help message and exit

</pre>
<p>Moreover <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> manages the case of missing arguments and of too many arguments.
This is only the tip of the iceberg: <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is able to do much more than that.</p>
</div>
<div class="section" id="scripts-with-default-arguments">
<h2><a class="toc-backref" href="#id18">Scripts with default arguments</a></h2>
<p>The need to have suitable defaults for command-line scripts is quite
common. For instance I have encountered this use case at work hundreds
of times:</p>
<pre class="literal-block">
# example4.py
from datetime import datetime

def main(dsn, table='product', today=datetime.today()):
    &quot;Do something on the database&quot;
    print(dsn, table, today)

if __name__ == '__main__': # manual management before argparse
    import sys
    args = sys.argv[1:]
    if not args:
        sys.exit('usage: python %s dsn' % sys.argv[0])
    elif len(args) &gt; 2:
        sys.exit('Unrecognized arguments: %s' % ' '.join(argv[2:]))
    main(*args)

</pre>
<p>Here I want to perform a query on a database table, by extracting the
most recent data: it makes sense for <tt class="docutils literal">today</tt> to be a default argument.
If there is a most used table (in this example a table called <tt class="docutils literal">'product'</tt>)
it also makes sense to make it a default argument. Performing the parsing
of the command-line arguments by hand takes 8 ugly lines of boilerplate
(using <a class="reference external" href="http://argparse.googlecode.com">argparse</a> would require about the same number of lines).
With <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> the entire <tt class="docutils literal">__main__</tt> block reduces to the usual two lines:</p>
<pre class="literal-block">
if __name__ == '__main__':
    import plac; plac.call(main)
</pre>
<p>In other words, six lines of boilerplate have been removed, and we get
the usage message for free:</p>
<pre class="literal-block">
usage: example5.py [-h] dsn [table] [today]

Do something on the database

positional arguments:
  dsn
  table       [product]
  today       [YYYY-MM-DD]

optional arguments:
  -h, --help  show this help message and exit

</pre>
<p>Notice that by default <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> prints the string representation
of the default values (with square brackets) in the usage message.
<a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> manages transparently even the case when you want to pass a
variable number of arguments. Here is an example, a script running
on a database a series of SQL scripts:</p>
<pre class="literal-block">
# example7.py
from datetime import datetime

def main(dsn, *scripts):
    &quot;Run the given scripts on the database&quot;
    for script in scripts:
        print('executing %s' % script)
        # ...

if __name__ == '__main__':
    import plac; plac.call(main)

</pre>
<p>Here is the usage message:</p>
<pre class="literal-block">
usage: example7.py [-h] dsn [scripts [scripts ...]]

Run the given scripts on the database

positional arguments:
  dsn
  scripts

optional arguments:
  -h, --help  show this help message and exit

</pre>
<p>The examples here should have made clear that <em>plac is able to figure out
the command-line arguments parser to use from the signature of the main
function</em>. This is the whole idea behind <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>: if the intent is clear,
let's the machine take care of the details.</p>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is inspired to an old Python Cookbook recipe of mine (<a class="reference external" href="http://code.activestate.com/recipes/278844-parsing-the-command-line/">optionparse</a>), in
the sense that it delivers the programmer from the burden of writing
the parser, but is less of a hack: instead of extracting the parser
from the docstring of the module, it extracts it from the signature of
the <tt class="docutils literal">main</tt> function.</p>
<p>The idea comes from the <cite>function annotations</cite> concept, a new
feature of Python 3. An example is worth a thousand words, so here
it is:</p>
<pre class="literal-block">
# example7_.py
from datetime import datetime

def main(dsn: &quot;Database dsn&quot;, *scripts: &quot;SQL scripts&quot;):
    &quot;Run the given scripts on the database&quot;
    for script in scripts:
        print('executing %s' % script)
        # ...

if __name__ == '__main__':
    import plac; plac.call(main)

</pre>
<p>Here the arguments of the <tt class="docutils literal">main</tt> function have been annotated with
strings which are intented to be used in the help message:</p>
<pre class="literal-block">
usage: example7_.py [-h] dsn [scripts [scripts ...]]

Run the given scripts on the database

positional arguments:
  dsn         Database dsn
  scripts     SQL scripts

optional arguments:
  -h, --help  show this help message and exit

</pre>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is able to recognize much more complex annotations, as
I will show in the next paragraphs.</p>
</div>
<div class="section" id="scripts-with-options-and-smart-options">
<h2><a class="toc-backref" href="#id19">Scripts with options (and smart options)</a></h2>
<p>It is surprising how few command-line scripts with options I have
written over the years (probably less than a hundred), compared to the
number of scripts with positional arguments I wrote (certainly more
than a thousand of them).  Still, this use case cannot be neglected.
The standard library modules (all of them) are quite verbose when it
comes to specifying the options and frankly I have never used them
directly. Instead, I have always relied on the
<a class="reference external" href="http://code.activestate.com/recipes/278844-parsing-the-command-line/">optionparse</a> recipe, which provides a convenient wrapper over
<a class="reference external" href="http://code.activestate.com/recipes/278844-parsing-the-command-line/">optionparse</a>. Alternatively, in the simplest cases, I have just
performed the parsing by hand. In <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> the parser is inferred by the
function annotations. Here is an example:</p>
<pre class="literal-block">
# example8.py
def main(command: (&quot;SQL query&quot;, 'option', 'q'), dsn):
    if command:
        print('executing %s on %s' % (command, dsn))
        # ...

if __name__ == '__main__':
    import plac; plac.call(main)

</pre>
<p>Here the argument <tt class="docutils literal">command</tt> has been annotated with the tuple
<tt class="docutils literal">(&quot;SQL query&quot;, 'option', 'c')</tt>: the first string is the help string
which will appear in the usage message, the second string tells <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>
that <tt class="docutils literal">command</tt> is an option and the third string that there is also
a short form of the option <tt class="docutils literal"><span class="pre">-c</span></tt>, the long form being <tt class="docutils literal"><span class="pre">--command</span></tt>.
The usage message is the following:</p>
<pre class="literal-block">
usage: example8.py [-h] [-q COMMAND] dsn

positional arguments:
  dsn

optional arguments:
  -h, --help            show this help message and exit
  -q COMMAND, --command COMMAND
                        SQL query

</pre>
<p>Here are two examples of usage:</p>
<pre class="literal-block">
$ python3 example8.py -c&quot;select * from table&quot; dsn
executing select * from table on dsn

$ python3 example8.py --command=&quot;select * from table&quot; dsn
executing select * from table on dsn
</pre>
<p>The third argument in the function annotation can be omitted: in such
case it will be assumed to be <tt class="docutils literal">None</tt>. The consequence is that
the usual dichotomy between long and short options (GNU-style options)
disappears: we get <em>smart options</em>, which have the single character prefix
of short options and behave like both long and short options, since
they can be abbreviated. Here is an example featuring smart options:</p>
<pre class="literal-block">
# example6.py
def main(dsn, command: (&quot;SQL query&quot;, 'option')='select * from table'):
    print('executing %r on %s' % (command, dsn))

if __name__ == '__main__':
    import plac; plac.call(main)

</pre>
<pre class="literal-block">
usage: example6.py [-h] [-command select * from table] dsn

positional arguments:
  dsn

optional arguments:
  -h, --help            show this help message and exit
  -command select * from table
                        SQL query

</pre>
<p>The following are all valid invocations ot the script:</p>
<pre class="literal-block">
$ python3 example6.py -c &quot;select&quot; dsn
executing 'select' on dsn
$ python3 example6.py -com &quot;select&quot; dsn
executing 'select' on dsn
$ python3 example6.py -command=&quot;select&quot; dsn
executing 'select' on dsn
</pre>
<p>Notice that the form <tt class="docutils literal"><span class="pre">-command=SQL</span></tt> is recognized only for the full
option, not for its abbreviations:</p>
<pre class="literal-block">
$ python3 example6.py -com=&quot;select&quot; dsn
usage: example6.py [-h] [-command COMMAND] dsn
example6.py: error: unrecognized arguments: -com=select
</pre>
<p>If the option is not passed, the variable <tt class="docutils literal">command</tt>
will get the value <tt class="docutils literal">None</tt>. However, it is possible to specify a non-trivial
default. Here is an example:</p>
<pre class="literal-block">
# example8_.py
def main(dsn, command: (&quot;SQL query&quot;, 'option', 'q')='select * from table'):
    print('executing %r on %s' % (command, dsn))

if __name__ == '__main__':
    import plac; plac.call(main)

</pre>
<p>Notice that the default value appears in the help message:</p>
<pre class="literal-block">
usage: example8_.py [-h] [-q select * from table] dsn

positional arguments:
  dsn

optional arguments:
  -h, --help            show this help message and exit
  -q select * from table, --command select * from table
                        SQL query

</pre>
<p>When you run the script and you do not pass the <tt class="docutils literal"><span class="pre">-command</span></tt> option, the
default query will be executed:</p>
<pre class="literal-block">
$ python3 example8_.py dsn
executing 'select * from table' on dsn
</pre>
</div>
<div class="section" id="scripts-with-flags">
<h2><a class="toc-backref" href="#id20">Scripts with flags</a></h2>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is able to recognize flags, i.e. boolean options which are
<tt class="docutils literal">True</tt> if they are passed to the command line and <tt class="docutils literal">False</tt>
if they are absent. Here is an example:</p>
<pre class="literal-block">
# example9.py

def main(verbose: ('prints more info', 'flag', 'v'), dsn: 'connection string'):
    if verbose:
        print('connecting to %s' % dsn)
    # ...

if __name__ == '__main__':
    import plac; plac.call(main)

</pre>
<pre class="literal-block">
usage: example9.py [-h] [-v] dsn

positional arguments:
  dsn            connection string

optional arguments:
  -h, --help     show this help message and exit
  -v, --verbose  prints more info

</pre>
<pre class="literal-block">
$ python3 example9.py -v dsn
connecting to dsn
</pre>
<p>Notice that it is an error trying to specify a default for flags: the
default value for a flag is always <tt class="docutils literal">False</tt>. If you feel the need to
implement non-boolean flags, you should use an option with two
choices, as explained in the &quot;more features&quot; section.</p>
<p>For consistency with the way the usage message is printed, I suggest
you to follow the Flag-Option-Required-Default (FORD) convention: in
the <tt class="docutils literal">main</tt> function write first the flag arguments, then the option
arguments, then the required arguments and finally the default
arguments. This is just a convention and you are not forced to use it,
except for the default arguments (including the varargs) which must
stay at the end as it is required by the Python syntax.</p>
<p>I also suggests to specify a one-character abbreviation for flags: in
this way you can use the GNU-style composition of flags (i.e. <tt class="docutils literal"><span class="pre">-zxvf</span></tt>
is an abbreviation of <tt class="docutils literal"><span class="pre">-z</span> <span class="pre">-x</span> <span class="pre">-v</span> <span class="pre">-f</span></tt>). I usually do not provide
the one-character abbreviation for options, since it does not make sense
to compose them.</p>
<p>Starting from <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> 0.9.1 dashes in options and flags are
automatically turned into underscores. This feature was implemented at user
request, to make it possible to use a more traditional naming. For
instance now you can have a <tt class="docutils literal"><span class="pre">--dry-run</span></tt> flag, whereas before you had
to use <tt class="docutils literal"><span class="pre">--dry_run</span></tt>.</p>
<pre class="literal-block">
def main(dry_run: ('Dry run', 'flag', 'd')):
    if dry_run:
        print('Doing nothing')
    else:
        print('Doing something')

if __name__ == '__main__':
    import plac; plac.call(main)

</pre>
<p>Here is an example of usage:</p>
<pre class="literal-block">
$ python3.2 dry_run.py -h
usage: dry_run.py [-h] [-d]

optional arguments:
  -h, --help     show this help message and exit
  -d, --dry-run  Dry run
</pre>
</div>
<div class="section" id="plac-for-python-2-x-users">
<h2><a class="toc-backref" href="#id21">plac for Python 2.X users</a></h2>
<p>I do not use Python 3. At work we just migrated to Python 2.7. It will
take years before we think to migrate to Python 3. I am pretty much
sure most Pythonistas are in the same situation. Therefore <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>
provides a way to work with function annotations even in Python 2.X
(including Python 2.3).  There is no magic involved; you just need to
add the annotations by hand. For instance the annotated function
declaration</p>
<pre class="literal-block">
def main(dsn: &quot;Database dsn&quot;, *scripts: &quot;SQL scripts&quot;):
    ...
</pre>
<p>is equivalent to the following code:</p>
<pre class="literal-block">
def main(dsn, *scripts):
    ...
main.__annotations__ = dict(
    dsn=&quot;Database dsn&quot;,
    scripts=&quot;SQL scripts&quot;)
</pre>
<p>One should be careful to match the keys of the annotation dictionary
with the names of the arguments in the annotated function; for lazy
people with Python 2.4 available the simplest way is to use the
<tt class="docutils literal">plac.annotations</tt> decorator that performs the check for you:</p>
<pre class="literal-block">
&#64;plac.annotations(
    dsn=&quot;Database dsn&quot;,
    scripts=&quot;SQL scripts&quot;)
def main(dsn, *scripts):
    ...
</pre>
<p>In the rest of this article I will assume that you are using Python 2.X with
X &gt;= 4 and I will use the <tt class="docutils literal">plac.annotations</tt> decorator. Notice however
that the core features of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> run even on Python 2.3.</p>
</div>
<div class="section" id="more-features">
<h2><a class="toc-backref" href="#id22">More features</a></h2>
<p>One of the goals of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is to have a learning curve of <em>minutes</em> for
its core features, compared to the learning curve of <em>hours</em> of
<a class="reference external" href="http://argparse.googlecode.com">argparse</a>. In order to reach this goal, I have <em>not</em> sacrificed all
the features of <a class="reference external" href="http://argparse.googlecode.com">argparse</a>. Actually a lot of the <a class="reference external" href="http://argparse.googlecode.com">argparse</a> power persists
in <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>.  Until now, I have only showed simple annotations, but in
general an annotation is a 6-tuple of the form</p>
<blockquote>
<tt class="docutils literal">(help, kind, abbrev, type, choices, metavar)</tt></blockquote>
<p>where <tt class="docutils literal">help</tt> is the help message, <tt class="docutils literal">kind</tt> is a string in the set {
<tt class="docutils literal">&quot;flag&quot;</tt>, <tt class="docutils literal">&quot;option&quot;</tt>, <tt class="docutils literal">&quot;positional&quot;</tt>}, <tt class="docutils literal">abbrev</tt> is a
one-character string or <tt class="docutils literal">None</tt>, <tt class="docutils literal">type</tt> is a callable taking a
string in input,
<tt class="docutils literal">choices</tt> is a discrete sequence of values and <tt class="docutils literal">metavar</tt> is a string.</p>
<p><tt class="docutils literal">type</tt> is used to automagically convert the command line arguments
from the string type to any Python type; by default there is no
conversion and <tt class="docutils literal">type=None</tt>.</p>
<p><tt class="docutils literal">choices</tt> is used to restrict the number of the valid
options; by default there is no restriction i.e. <tt class="docutils literal">choices=None</tt>.</p>
<p><tt class="docutils literal">metavar</tt> has two meanings. For a positional argument it is used to
change the argument name in the usage message (and only there). By
default the metavar is <tt class="docutils literal">None</tt> and the name in the usage message is
the same as the argument name.  For an option
the <tt class="docutils literal">metavar</tt> is used differently in the usage message, which has
now the form <tt class="docutils literal"><span class="pre">[--option-name</span> METAVAR]</tt>. If the <tt class="docutils literal">metavar</tt> is <tt class="docutils literal">None</tt>,
then it is equal to the uppercased name of the argument, unless the
argument has a default: then it is equal to the stringified
form of the default.</p>
<p>Here is an example showing many of the features (copied from the
<a class="reference external" href="http://argparse.googlecode.com">argparse</a> documentation):</p>
<pre class="literal-block">
# example10.py
import plac

&#64;plac.annotations(
operator=(&quot;The name of an operator&quot;, 'positional', None, str, ['add', 'mul']),
numbers=(&quot;A number&quot;, 'positional', None, float, None, &quot;n&quot;))
def main(operator, *numbers):
    &quot;A script to add and multiply numbers&quot;
    if operator == 'mul':
        op = float.__mul__
        result = 1.0
    else: # operator == 'add'
        op = float.__add__
        result = 0.0
    for n in numbers:
        result = op(result, n)
    return result

if __name__ == '__main__':
    print(plac.call(main))

</pre>
<p>Here is the usage:</p>
<pre class="literal-block">
usage: example10.py [-h] {add,mul} [n [n ...]]

A script to add and multiply numbers

positional arguments:
  {add,mul}   The name of an operator
  n           A number

optional arguments:
  -h, --help  show this help message and exit

</pre>
<p>Notice that the docstring of the <tt class="docutils literal">main</tt> function has been automatically added
to the usage message. Here are a couple of examples of usage:</p>
<pre class="literal-block">
$ python example10.py add 1 2 3 4
10.0
$ python example10.py mul 1 2 3 4
24.0
$ python example10.py ad 1 2 3 4 # a mispelling error
usage: example10.py [-h] {add,mul} [n [n ...]]
example10.py: error: argument operator: invalid choice: 'ad' (choose from 'add', 'mul')
</pre>
<p><tt class="docutils literal">plac.call</tt> can also be used in doctests like this:</p>
<pre class="doctest-block">
&gt;&gt;&gt; import plac, example10
&gt;&gt;&gt; plac.call(example10.main, ['add', '1', '2'])
3.0
</pre>
<p><tt class="docutils literal">plac.call</tt> works for generators too:</p>
<pre class="doctest-block">
&gt;&gt;&gt; def main(n):
...     for i in range(int(n)):
...         yield i
&gt;&gt;&gt; plac.call(main, ['3'])
[0, 1, 2]
</pre>
<p>Internally <tt class="docutils literal">plac.call</tt> tries to convert the output of the main function
into a list, if possible. If the output is not iterable or it is a
string, it is left unchanged, but if it is iterable it is converted.
In particular, generator objects are exhausted by <tt class="docutils literal">plac.call</tt>.</p>
<p>This behavior avoids mistakes like forgetting of applying
<tt class="docutils literal">list(result)</tt> to the result of <tt class="docutils literal">plac.call</tt>; moreover it makes
errors visible early, and avoids mistakes in code like the following:</p>
<pre class="literal-block">
try:
    result = plac.call(main, args)
except:
   # do something
</pre>
<p>Without eagerness, a main function returning a generator object would
not raise any exception until the generator is iterated over.
If you are a fan of lazyness, you can still have it by setting the <tt class="docutils literal">eager</tt>
flag to <tt class="docutils literal">False</tt>, as in the following example:</p>
<pre class="literal-block">
for line in plac.call(main, args, eager=False):
    print(line)
</pre>
<p>If <tt class="docutils literal">main</tt> returns a generator object this example will print each
line as soon as available, whereas the default behaviour is to print
all the lines together and the end of the computation.</p>
</div>
<div class="section" id="a-realistic-example">
<h2><a class="toc-backref" href="#id23">A realistic example</a></h2>
<p>Here is a more realistic script using most of the features of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> to
run SQL queries on a database by relying on <a class="reference external" href="http://www.sqlalchemy.org/">SQLAlchemy</a>. Notice the usage
of the <tt class="docutils literal">type</tt> feature to automagically convert a SQLAlchemy connection
string into a <a class="reference external" href="http://www.sqlalchemy.org/docs/reference/ext/sqlsoup.html">SqlSoup</a> object:</p>
<pre class="literal-block">
# dbcli.py
import plac
from sqlalchemy.ext.sqlsoup import SqlSoup

&#64;plac.annotations(
    db=plac.Annotation(&quot;Connection string&quot;, type=SqlSoup),
    header=plac.Annotation(&quot;Header&quot;, 'flag', 'H'),
    sqlcmd=plac.Annotation(&quot;SQL command&quot;, 'option', 'c', str, metavar=&quot;SQL&quot;),
    delimiter=plac.Annotation(&quot;Column separator&quot;, 'option', 'd'),
    scripts=plac.Annotation(&quot;SQL scripts&quot;),
    )
def main(db, header, sqlcmd, delimiter=&quot;|&quot;, *scripts):
    &quot;A script to run queries and SQL scripts on a database&quot;
    yield 'Working on %s' % db.bind.url

    if sqlcmd:
        result = db.bind.execute(sqlcmd)
        if header: # print the header
            yield delimiter.join(result.keys())
        for row in result: # print the rows
            yield delimiter.join(map(str, row))

    for script in scripts:
        db.bind.execute(open(script).read())
        yield 'executed %s' % script

if __name__ == '__main__':
    for output in plac.call(main):
        print(output)

</pre>
<p>You can see the <em>yield-is-print</em> pattern here: instead of using
<tt class="docutils literal">print</tt> in the main function, I use <tt class="docutils literal">yield</tt>, and I perform the
print in the <tt class="docutils literal">__main__</tt> block. The advantage of the pattern is that
tests invoking <tt class="docutils literal">plac.call</tt> and checking the result become trivial:
had I performed the printing in the main function, the test would have
involved an ugly hack like redirecting <tt class="docutils literal">sys.stdout</tt> to a
<tt class="docutils literal">StringIO</tt> object.</p>
<p>Here is the usage message:</p>
<pre class="literal-block">
usage: dbcli.py [-h] [-H] [-c SQL] [-d |] db [scripts [scripts ...]]

A script to run queries and SQL scripts on a database

positional arguments:
  db                    Connection string
  scripts               SQL scripts

optional arguments:
  -h, --help            show this help message and exit
  -H, --header          Header
  -c SQL, --sqlcmd SQL  SQL command
  -d |, --delimiter |   Column separator

</pre>
<p>You can check for yourself that the script works.</p>
</div>
<div class="section" id="keyword-arguments">
<h2><a class="toc-backref" href="#id24">Keyword arguments</a></h2>
<p>Starting from release 0.4, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> supports keyword arguments.  In
practice that means that if your main function has keyword arguments,
<a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> treats specially arguments of the form <tt class="docutils literal">&quot;name=value&quot;</tt> in the
command line.  Here is an example:</p>
<pre class="literal-block">
# example12.py
import plac

&#64;plac.annotations(
   opt=('some option', 'option'),
   args='default arguments',
   kw='keyword arguments')
def main(opt, *args, **kw):
   if opt:
      yield 'opt=%s' % opt
   if args:
      yield 'args=%s' % str(args)
   if kw:
      yield 'kw=%s' % kw

if __name__ == '__main__':
    for output in plac.call(main):
       print(output)

</pre>
<p>Here is the generated usage message:</p>
<pre class="literal-block">
usage: example12.py [-h] [-opt OPT] [args [args ...]] [kw [kw ...]]

positional arguments:
  args        default arguments
  kw          keyword arguments

optional arguments:
  -h, --help  show this help message and exit
  -opt OPT    some option

</pre>
<p>Here is how you call the script:</p>
<pre class="literal-block">
$ python example12.py -o X a1 a2 name=value
opt=X
args=('a1', 'a2')
kw={'name': 'value'}
</pre>
<p>When using keyword arguments, one must be careful to use names which
are not alreay taken; for instance in this examples the name <tt class="docutils literal">opt</tt>
is taken:</p>
<pre class="literal-block">
$ python example12.py 1 2 kw1=1 kw2=2 opt=0
usage: example12.py [-h] [-o OPT] [args [args ...]] [kw [kw ...]]
example12.py: error: colliding keyword arguments: opt
</pre>
<p>The names taken are the names of the flags, of the options, and of the
positional arguments, excepted varargs and keywords. This limitation
is a consequence of the way the argument names are managed in function calls
by the Python language.</p>
</div>
<div class="section" id="plac-vs-argparse">
<h2><a class="toc-backref" href="#id25">plac vs argparse</a></h2>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is opinionated and by design it does not try to make available
all of the features of <a class="reference external" href="http://argparse.googlecode.com">argparse</a> in an easy way.  In particular you
should be aware of the following limitations/differences (the
following assumes knowledge of <a class="reference external" href="http://argparse.googlecode.com">argparse</a>):</p>
<ul class="simple">
<li>plac does not support the destination concept: the destination
coincides with the name of the argument, always. This restriction
has some drawbacks. For instance, suppose you want to define a long
option called <tt class="docutils literal"><span class="pre">--yield</span></tt>. In this case the destination would be <tt class="docutils literal">yield</tt>,
which is a Python keyword, and since you cannot introduce an
argument with that name in a function definition, it is impossible
to implement it. Your choices are to change the name of the long
option, or to use <a class="reference external" href="http://argparse.googlecode.com">argparse</a> with a suitable destination.</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> does not support &quot;required options&quot;. As the <a class="reference external" href="http://argparse.googlecode.com">argparse</a>
documentation puts it: <em>Required options are generally considered bad
form - normal users expect options to be optional. You should avoid
the use of required options whenever possible.</em> Notice that since
<a class="reference external" href="http://argparse.googlecode.com">argparse</a> supports them, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> can manage them too, but not directly.</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> supports only regular boolean flags. <a class="reference external" href="http://argparse.googlecode.com">argparse</a> has the ability to
define generalized two-value flags with values different from <tt class="docutils literal">True</tt>
and <tt class="docutils literal">False</tt>. An earlier version of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> had this feature too, but
since you can use options with two choices instead, and in any case
the conversion from <tt class="docutils literal">{True, False}</tt> to any couple of values
can be trivially implemented with a ternary operator
(<tt class="docutils literal">value1 if flag else value2</tt>), I have removed it (KISS rules!).</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> does not support <tt class="docutils literal">nargs</tt> options directly (it uses them internally,
though, to implement flag recognition). The reason it that all the use
cases of interest to me are covered by <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> and I did not feel the need
to increase the learning curve by adding direct support for <tt class="docutils literal">nargs</tt>.</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> does support subparsers, but you must read the <a class="reference external" href="in-writing">advanced usage
document</a> to see how it works.</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> does not support actions directly. This also
looks like a feature too advanced for the goals of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>. Notice however
that the ability to define your own annotation objects (again, see
the  <a class="reference external" href="in-writing">advanced usage document</a>) may mitigate the need for custom actions.</li>
</ul>
<p>On the plus side, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> can leverage directly on a number of <a class="reference external" href="http://argparse.googlecode.com">argparse</a> features.</p>
<p>For instance, you can use <a class="reference external" href="http://argparse.googlecode.com/svn/tags/r11/doc/other-utilities.html?highlight=filetype#FileType">argparse.FileType</a> directly. Moreover,
it is possible to pass options to the underlying
<tt class="docutils literal">argparse.ArgumentParser</tt> object (currently it accepts the default
arguments <tt class="docutils literal">description</tt>, <tt class="docutils literal">epilog</tt>, <tt class="docutils literal">prog</tt>, <tt class="docutils literal">usage</tt>,
<tt class="docutils literal">add_help</tt>, <tt class="docutils literal">argument_default</tt>, <tt class="docutils literal">parents</tt>, <tt class="docutils literal">prefix_chars</tt>,
<tt class="docutils literal">fromfile_prefix_chars</tt>, <tt class="docutils literal">conflict_handler</tt>, <tt class="docutils literal">formatter_class</tt>).
It is enough to set such attributes on the <tt class="docutils literal">main</tt> function.  For
instance writing</p>
<pre class="literal-block">
def main(...):
    pass

main.add_help = False
</pre>
<p>disables the recognition of the help flag <tt class="docutils literal"><span class="pre">-h,</span> <span class="pre">--help</span></tt>. This
mechanism does not look particularly elegant, but it works well
enough.  I assume that the typical user of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> will be happy with
the defaults and would not want to change them; still it is possible
if she wants to.</p>
<p>For instance, by setting the <tt class="docutils literal">description</tt> attribute, it is possible
to add a comment to the usage message (by default the docstring of the
<tt class="docutils literal">main</tt> function is used as description).</p>
<p>It is also possible to change the option prefix; for
instance if your script must run under Windows and you want to use &quot;/&quot;
as option prefix you can add the line:</p>
<pre class="literal-block">
main.prefix_chars='/-'
</pre>
<p>The first prefix char (<tt class="docutils literal">/</tt>) is used
as the default for the recognition of options and flags;
the second prefix char (<tt class="docutils literal">-</tt>) is kept to keep the <tt class="docutils literal"><span class="pre">-h/--help</span></tt> option
working: however you can disable it and reimplement it, if you like.</p>
<p>It is possible to access directly the underlying <a class="reference external" href="http://argparse.googlecode.com/svn/tags/r11/doc/ArgumentParser.html">ArgumentParser</a> object, by
invoking the <tt class="docutils literal">plac.parser_from</tt> utility function:</p>
<pre class="doctest-block">
&gt;&gt;&gt; import plac
&gt;&gt;&gt; def main(arg):
...     pass
...
&gt;&gt;&gt; print(plac.parser_from(main)) #doctest: +ELLIPSIS
ArgumentParser(prog=...)
</pre>
<p>Internally <tt class="docutils literal">plac.call</tt> uses <tt class="docutils literal">plac.parser_from</tt>. Notice that when
<tt class="docutils literal">plac.call(func)</tt> is invoked multiple time, the parser is re-used
and not rebuilt from scratch again.</p>
<p>I use <tt class="docutils literal">plac.parser_from</tt> in the unit tests of the module, but regular
users should not need to use it, unless they want to access <em>all</em>
of the features of <a class="reference external" href="http://argparse.googlecode.com">argparse</a> directly without calling the main function.</p>
<p>Interested readers should read the documentation of <a class="reference external" href="http://argparse.googlecode.com">argparse</a> to
understand the meaning of the other options. If there is a set of
options that you use very often, you may consider writing a decorator
adding such options to the <tt class="docutils literal">main</tt> function for you. For simplicity,
<a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> does not perform any magic.</p>
</div>
<div class="section" id="final-example-a-shelve-interface">
<h2><a class="toc-backref" href="#id26">Final example: a shelve interface</a></h2>
<p>Here is a nontrivial example showing off many <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> feature, including
keyword arguments recognition.
The use case is the following: suppose we have stored the
configuration parameters of a given application into a Python shelve
and we need a command-line tool to edit the shelve.
A possible implementation using <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> could be the following:</p>
<pre class="literal-block">
# ishelve.py
import os, shelve, plac

DEFAULT_SHELVE = os.path.expanduser('~/conf.shelve')

&#64;plac.annotations(
    help=('show help', 'flag'),
    showall=('show all parameters in the shelve', 'flag'),
    clear=('clear the shelve', 'flag'),
    delete=('delete an element', 'option'),
    filename=('filename of the shelve', 'option'),
    params='names of the parameters in the shelve',
    setters='setters param=value')
def main(help, showall, clear, delete, filename=DEFAULT_SHELVE,
         *params, **setters):
    &quot;A simple interface to a shelve. Use .help to see the available commands.&quot;
    sh = shelve.open(filename)
    try:
        if not any([help, showall, clear, delete, params, setters]):
            yield 'no arguments passed, use .help to see the available commands'
        elif help: # custom help
            yield 'Commands: .help, .showall, .clear, .delete'
            yield '&lt;param&gt; ...'
            yield '&lt;param=value&gt; ...'
        elif showall:
            for param, name in sh.items():
                yield '%s=%s' % (param, name)
        elif clear:
            sh.clear()
            yield 'cleared the shelve'
        elif delete:
            try:
                del sh[delete]
            except KeyError:
                yield '%s: not found' % delete
            else:
                yield 'deleted %s' % delete
        for param in params:
            try:
                yield sh[param]
            except KeyError:
                yield '%s: not found' % param           
        for param, value in setters.items():
            sh[param] = value
            yield 'setting %s=%s' % (param, value)
    finally:
        sh.close()

main.add_help = False # there is a custom help, remove the default one
main.prefix_chars = '.' # use dot-prefixed commands

if __name__ == '__main__':
    for output in plac.call(main):
        print(output)

</pre>
<p>A few notes are in order:</p>
<ol class="arabic simple">
<li>I have disabled the ordinary help provided by <a class="reference external" href="http://argparse.googlecode.com">argparse</a> and I have
implemented a custom help command.</li>
<li>I have changed the prefix character used to recognize the options
to a dot.</li>
<li>Keyword arguments recognition (in the <tt class="docutils literal">**setters</tt>) is used to make it
possible to store a value in the shelve with the syntax
<tt class="docutils literal">param_name=param_value</tt>.</li>
<li><tt class="docutils literal">*params</tt> are used to retrieve parameters from the shelve and some
error checking is performed in the case of missing parameters</li>
<li>A command to clear the shelve is implemented as a flag (<tt class="docutils literal">.clear</tt>).</li>
<li>A command to delete a given parameter is implemented as an option
(<tt class="docutils literal">.delete</tt>).</li>
<li>There is an option with default (<tt class="docutils literal">.filename=conf.shelve</tt>) to store
the filename of the shelve.</li>
<li>All things considered, the code looks like a poor man object oriented
interface implemented with a chain of elifs instead of methods. Of course,
<a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> can do better than that, but let me start from a low-level approach
first.</li>
</ol>
<p>If you run <tt class="docutils literal">ishelve.py</tt> without arguments you get the following
message:</p>
<pre class="literal-block">
$ python ishelve.py
no arguments passed, use .help to see the available commands
</pre>
<p>If you run <tt class="docutils literal">ishelve.py</tt> with the option <tt class="docutils literal">.h</tt> (or any abbreviation
of <tt class="docutils literal">.help</tt>) you get:</p>
<pre class="literal-block">
$ python ishelve.py .h
Commands: .help, .showall, .clear, .delete
&lt;param&gt; ...
&lt;param=value&gt; ...
</pre>
<p>You can check by hand that the tool work:</p>
<pre class="literal-block">
$ python ishelve.py .clear # start from an empty shelve
cleared the shelve
$ python ishelve.py a=1 b=2
setting a=1
setting b=2
$ python ishelve.py .showall
b=2
a=1
$ python ishelve.py .del b # abbreviation for .delete
deleted b
$ python ishelve.py a
1
$ python ishelve.py b
b: not found
$ python ishelve.py .cler # mispelled command
usage: ishelve.py [.help] [.showall] [.clear] [.delete DELETE]
                  [.filename /home/micheles/conf.shelve]
                  [params [params ...]] [setters [setters ...]]
ishelve.py: error: unrecognized arguments: .cler
</pre>
</div>
<div class="section" id="plac-vs-the-rest-of-the-world">
<h2><a class="toc-backref" href="#id27">plac vs the rest of the world</a></h2>
<p>Originally <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> boasted about being &quot;the easiest command-line
arguments parser in the world&quot;. Since then, people started pointing
out to me various projects which are based on the same idea
(extracting the parser from the main function signature) and are
arguably even easier than <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>:</p>
<ul class="simple">
<li><a class="reference external" href="http://pypi.python.org/pypi/opterator">opterator</a> by Dusty Phillips</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/CLIArgs">CLIArgs</a> by Pavel Panchekha</li>
<li><a class="reference external" href="http://pypi.python.org/pypi/commandline">commandline</a> by David Laban</li>
</ul>
<p>Luckily for me none of such projects had the idea of using
function annotations and <a class="reference external" href="http://argparse.googlecode.com">argparse</a>; as a consequence, they are
no match for the capabilities of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>.</p>
<p>Of course, there are tons of other libraries to parse the command
line. For instance <a class="reference external" href="http://pypi.python.org/pypi/Clap/0.7">Clap</a> by Matthew Frazier which appeared on PyPI
just the day before <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>; <a class="reference external" href="http://pypi.python.org/pypi/Clap/0.7">Clap</a> is fine but it is certainly not
easier than <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>.</p>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> can also be used as a replacement of the <a class="reference external" href="http://docs.python.org/library/cmd.html">cmd</a> module in the standard
library and as such it shares many features with the module <a class="reference external" href="http://packages.python.org/cmd2/">cmd2</a> by
Catherine Devlin. However, this is completely coincidental, since I became
aware of the <a class="reference external" href="http://packages.python.org/cmd2/">cmd2</a> module only after writing <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>.</p>
<p>Command-line argument parsers keep coming out; between the newcomers I
will notice <a class="reference external" href="https://github.com/pulp/marrow.script">marrow.script</a> by Alice Bevan-McGregor, which is quite
similar to <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> in spirit, but does not rely on <a class="reference external" href="http://argparse.googlecode.com">argparse</a> at all.
<a class="reference external" href="http://packages.python.org/argh">Argh</a> by Andrey Mikhaylenko is also worth mentioning: it is based
on <a class="reference external" href="http://argparse.googlecode.com">argparse</a>, it came after <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> and I must give credit to the author
for the choice of the name, much funnier than plac!</p>
</div>
<div class="section" id="the-future">
<h2><a class="toc-backref" href="#id28">The future</a></h2>
<p>Currently the core of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is around 200 lines of code, not counting blanks,
comments and docstrings. I do not plan to extend the core much in the
future. The idea is to keep the module short: it is and it should
remain a little wrapper over <a class="reference external" href="http://argparse.googlecode.com">argparse</a>. Actually I have thought about
contributing the core back to <a class="reference external" href="http://argparse.googlecode.com">argparse</a> if <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> becomes successfull
and gains a reasonable number of users. For the moment it should be
considered in a frozen status.</p>
<p>Notice that even if <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> has been designed to be simple to use for
simple stuff, its power should not be underestimated; it is actually a
quite advanced tool with a domain of applicability which far exceeds
the realm of command-line arguments parsers.</p>
<p>Version 0.5 of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> doubled the code base and the documentation: it is
based on the idea of using <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> to implement command-line interpreters,
i.e. something akin to the <tt class="docutils literal">cmd</tt> module in the standard library, only better.
The new features of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> are described in the <a class="reference external" href="in-writing">advanced usage document</a> .
They are implemented in a separated module (<tt class="docutils literal">plac_ext.py</tt>), since
they require Python 2.5 to work, whereas <tt class="docutils literal">plac_core.py</tt> only requires
Python 2.3.</p>
</div>
<div class="section" id="trivia-the-story-behind-the-name">
<h2><a class="toc-backref" href="#id29">Trivia: the story behind the name</a></h2>
<p>The <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> project started very humbly: I just wanted to make
easy_installable my old <a class="reference external" href="http://code.activestate.com/recipes/278844-parsing-the-command-line/">optionparse</a> recipe, and to publish it on PyPI.
The original name of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> was optionparser and the idea behind it was
to build an <a class="reference external" href="http://docs.python.org/library/optparse.html?highlight=optionparser#optparse.OptionParser">OptionParser</a> object from the docstring of the module.
However, before doing that, I decided to check out the <a class="reference external" href="http://argparse.googlecode.com">argparse</a> module,
since I knew it was going into Python 2.7 and Python 2.7 was coming out.
Soon enough I realized two things:</p>
<ol class="arabic simple">
<li>the single greatest idea of <a class="reference external" href="http://argparse.googlecode.com">argparse</a> was unifying the positional arguments
and the options in a single namespace object;</li>
<li>parsing the docstring was so old-fashioned, considering the existence
of functions annotations in Python 3.</li>
</ol>
<p>Putting together these two observations with the original idea of inferring the
parser I decided to build an <a class="reference external" href="http://argparse.googlecode.com/svn/tags/r11/doc/ArgumentParser.html">ArgumentParser</a> object from function
annotations. The <tt class="docutils literal">optionparser</tt> name was ruled out, since I was
now using <a class="reference external" href="http://argparse.googlecode.com">argparse</a>; a name like <tt class="docutils literal">argparse_plus</tt> was also ruled out,
since the typical usage was completely different from the <a class="reference external" href="http://argparse.googlecode.com">argparse</a> usage.</p>
<p>I made a research on PyPI and the name <em>clap</em> (Command Line Arguments Parser)
was not taken, so I renamed everything to clap. After two days
a <a class="reference external" href="http://pypi.python.org/pypi/Clap/0.7">Clap</a> module appeared on PyPI &lt;expletives deleted&gt;!</p>
<p>Having little imagination, I decided to rename everything again to plac,
an anagram of clap: since it is a non-existing English name, I hope nobody
will steal it from me!</p>
<p>That concludes the section about the basic usage of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>. You are now ready to
read about the advanced usage.</p>
</div>
</div>
<div class="section" id="advanced-usages-of-plac">
<h1><a class="toc-backref" href="#id30">Advanced usages of plac</a></h1>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id31">Introduction</a></h2>
<p>One of the design goals of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is to make it dead easy to write a
scriptable and testable interface for an application.  You can use
<a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> whenever you have an API with strings in input and strings in
output, and that includes a <em>huge</em> domain of applications.</p>
<p>A string-oriented interface is a scriptable interface by
construction. That means that you can define a command language for
your application and that it is possible to write scripts which are
interpretable by <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> and can be run as batch scripts.</p>
<p>Actually, at the most general level, you can see <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> as a generic tool to
write domain specific languages (DSL). With <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> you
can test your application interactively as well as with batch
scripts, and even with the analogous of Python doctests for your
defined language.</p>
<p>You can easily replace the <tt class="docutils literal">cmd</tt> module of the standard library and
you could easily write an application like <a class="reference external" href="http://twill.idyll.org/">twill</a> with <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>. Or you
could use it to script your building procedure. <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> also supports
parallel execution of multiple commands and can be used as
task manager. It is also quite easy to build a GUI
or a Web application on top of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>. When speaking of things
you can do with <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>, your imagination is the only limit!</p>
</div>
<div class="section" id="from-scripts-to-interactive-applications">
<h2><a class="toc-backref" href="#id32">From scripts to interactive applications</a></h2>
<p>Command-line scripts have many advantages, but they are no substitute
for interactive applications.
In particular, if you have a script with a large startup time which must be run
multiple times, it is best to turn it into an interactive application,
so that the startup is performed only once.  <tt class="docutils literal">plac</tt> provides an
<tt class="docutils literal">Interpreter</tt> class just for this purpose.</p>
<p>The <tt class="docutils literal">Interpreter</tt> class wraps the main function of a script and
provides an <tt class="docutils literal">.interact</tt> method to start an interactive interpreter
reading commands from the console.</p>
<p>For instance, you can define an interactive interpreter on top of the
<tt class="docutils literal">ishelve</tt> script introduced in the <a class="reference external" href="http://plac.googlecode.com/hg/doc/plac.html">basic documentation</a> as
follows:</p>
<pre class="literal-block">
# shelve_interpreter.py
import plac, ishelve

&#64;plac.annotations(
    interactive=('start interactive interface', 'flag'),
    subcommands='the commands of the underlying ishelve interpreter')
def main(interactive, *subcommands):
    &quot;&quot;&quot;
    This script works both interactively and non-interactively.
    Use .help to see the internal commands.
    &quot;&quot;&quot;
    if interactive:
        plac.Interpreter(ishelve.main).interact()
    else:
        for out in plac.call(ishelve.main, subcommands):
            print(out)

if __name__ == '__main__':
    plac.call(main)

</pre>
<p>A trick has been used here: the ishelve command-line interface has been
hidden inside an external interface. They are distinct: for instance
the external interface recognizes the <tt class="docutils literal"><span class="pre">-h/--help</span></tt> flag whereas the
internal interface only recognizes the <tt class="docutils literal">.help</tt> command:</p>
<pre class="literal-block">
$ python shelve_interpreter.py -h
</pre>
<pre class="literal-block">
usage: shelve_interpreter.py [-h] [-interactive]
                             [subcommands [subcommands ...]]

    This script works both interactively and non-interactively.
    Use .help to see the internal commands.
    

positional arguments:
  subcommands   the commands of the underlying ishelve interpreter

optional arguments:
  -h, --help    show this help message and exit
  -interactive  start interactive interface

</pre>
<p>Thanks to this ingenuous trick, the script can be run both interactively
and non-interactively:</p>
<pre class="literal-block">
$ python shelve_interpreter.py .clear # non-interactive use
cleared the shelve
</pre>
<p>Here is an usage session:</p>
<pre class="literal-block">
$ python shelve_interpreter.py -i # interactive use
A simple interface to a shelve. Use .help to see the available commands.
i&gt; .help
Commands: .help, .showall, .clear, .delete
&lt;param&gt; ...
&lt;param=value&gt; ...
i&gt; a=1
setting a=1
i&gt; a
1
i&gt; b=2
setting b=2
i&gt; a b
1
2
i&gt; .del a
deleted a
i&gt; a
a: not found
i&gt; .show
b=2
i&gt; [CTRL-D]
</pre>
<p>The <tt class="docutils literal">.interact</tt> method
reads commands from the console and send them to the
underlying interpreter, until the user send a CTRL-D
command (CTRL-Z in Windows). There is a default
argument <tt class="docutils literal"><span class="pre">prompt='i&gt;</span> '</tt> which
can be used to change the prompt. The text displayed at the beginning
of the interactive session is the docstring of the main function.
<tt class="docutils literal">plac</tt> also understands command abbreviations: in this example
<tt class="docutils literal">del</tt> is an abbreviation for <tt class="docutils literal">delete</tt>. In case of ambiguous
abbreviations <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> raises a <tt class="docutils literal">NameError</tt>.</p>
<p>Finally I must notice that <tt class="docutils literal">plac.Interpreter</tt> is available only if you
are using a recent version of Python (&gt;= 2.5), because it is a context
manager object which uses extended generators internally.</p>
</div>
<div class="section" id="testing-a-plac-application">
<h2><a class="toc-backref" href="#id33">Testing a plac application</a></h2>
<p>You can conveniently test your application in interactive mode.
However manual testing is a poor substitute for automatic testing.</p>
<p>In principle, one could write automatic tests for the
<tt class="docutils literal">ishelve</tt> application by using <tt class="docutils literal">plac.call</tt> directly:</p>
<pre class="literal-block">
# test_ishelve.py
import plac, ishelve

def test():
    assert plac.call(ishelve.main, ['.clear']) == ['cleared the shelve']
    assert plac.call(ishelve.main, ['a=1']) == ['setting a=1']
    assert plac.call(ishelve.main, ['a']) == ['1']
    assert plac.call(ishelve.main, ['.delete=a']) == ['deleted a']
    assert plac.call(ishelve.main, ['a']) == ['a: not found']

if __name__ == '__main__':
    test()

</pre>
<p>However, using <tt class="docutils literal">plac.call</tt> is not especially nice. The big
issue is that <tt class="docutils literal">plac.call</tt> responds to invalid input by printing an
error message on stderr and by raising a <tt class="docutils literal">SystemExit</tt>: this is
certainly not a nice thing to do in a test.</p>
<p>As a consequence of this behavior it is impossible to test for invalid
commands, unless you wrap the <tt class="docutils literal">SystemExit</tt> exception by
hand each time (a possibly you do something with the error message in
stderr too). Luckily, <tt class="docutils literal">plac</tt> offers a better testing support through
the <tt class="docutils literal">check</tt> method of <tt class="docutils literal">Interpreter</tt> objects:</p>
<pre class="literal-block">
# test_ishelve_more.py
from __future__ import with_statement
import plac, ishelve

def test():
    with plac.Interpreter(ishelve.main) as i:
        i.check('.clear', 'cleared the shelve')
        i.check('a=1', 'setting a=1')
        i.check('a', '1')
        i.check('.delete=a', 'deleted a')
        i.check('a', 'a: not found')

</pre>
<p>The method <tt class="docutils literal">.check(given_input, expected_output)</tt> works on strings
and raises an <tt class="docutils literal">AssertionError</tt> if the output produced by the
interpreter is different from the expected output for the given input.
Notice that <tt class="docutils literal">AssertionError</tt> is catched by tools like <tt class="docutils literal">py.test</tt> and
<tt class="docutils literal">nosetests</tt> and actually <tt class="docutils literal">plac</tt> tests are intended to be run with
such tools.</p>
<p>Interpreters offer a minor syntactic advantage with respect to calling
<tt class="docutils literal">plac.call</tt> directly, but they offer a <em>major</em> semantic advantage when things
go wrong (read exceptions): an <tt class="docutils literal">Interpreter</tt> object internally invokes
something like <tt class="docutils literal">plac.call</tt>, but it wraps all exceptions, so that <tt class="docutils literal">i.check</tt>
is guaranteed not to raise any exception except <tt class="docutils literal">AssertionError</tt>.</p>
<p>Even the <tt class="docutils literal">SystemExit</tt> exception is captured and you can write your test as</p>
<blockquote>
<tt class="docutils literal"><span class="pre">i.check('-cler',</span> 'SystemExit: unrecognized arguments: <span class="pre">-cler')</span></tt></blockquote>
<p>without risk of exiting from the Python interpreter.</p>
<p>There is a second advantage of interpreters: if the main function
contains some initialization code and finalization code (<tt class="docutils literal">__enter__</tt>
and <tt class="docutils literal">__exit__</tt> functions) they will be run at the beginning and at
the end of the interpreter loop, whereas <tt class="docutils literal">plac.call</tt> ignores
the initialization/finalization code.</p>
</div>
<div class="section" id="plac-easy-tests">
<h2><a class="toc-backref" href="#id34">Plac easy tests</a></h2>
<p>Writing your tests in terms of <tt class="docutils literal">Interpreter.check</tt> is certainly an
improvement over writing them in terms of <tt class="docutils literal">plac.call</tt>, but they
are still too low-level for my taste. The <tt class="docutils literal">Interpreter</tt> class provides
support for doctest-style tests, a.k.a. <em>plac easy tests</em>.</p>
<p>By using plac easy tests you can cut and paste your interactive session and
turn it into a runnable automatics test.
Consider for instance the following file <tt class="docutils literal">ishelve.placet</tt> (the <tt class="docutils literal">.placet</tt>
extension is a mnemonic for plac easy tests):</p>
<pre class="literal-block">
#!ishelve.py
i&gt; .clear # start from a clean state
cleared the shelve
i&gt; a=1
setting a=1
i&gt; a
1
i&gt; .del a
deleted a
i&gt; a
a: not found
i&gt; .cler # spelling error
.cler: not found

</pre>
<p>Notice the precence of the shebang line containing the name of the
<a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> tool to test (a <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> tool is just a Python module with a
function called <tt class="docutils literal">main</tt>). The shebang is ignored by the interpreter
(it looks like a comment to it) but it is there so that external
tools (say a test runner) can infer the plac interpreter
to use to test the file.</p>
<p>You can run the <tt class="docutils literal">ishelve.placet</tt> file by calling the
<tt class="docutils literal">.doctest</tt> method of the interpreter, as in this example:</p>
<pre class="literal-block">
$ python -c&quot;import plac, ishelve
plac.Interpreter(ishelve.main).doctest(open('ishelve.placet'), verbose=True)&quot;
</pre>
<p>Internally <tt class="docutils literal">Interpreter.doctests</tt> invokes something like <tt class="docutils literal">Interpreter.check</tt>
multiple times inside the same context and compare the output with the
expected output: if even a check fails, the whole test fail.</p>
<p>You should realize tha the easy tests supported by <tt class="docutils literal">plac</tt> are <em>not</em>
unittests: they are functional tests. They model the user interaction and the
order of the operations generally matters.  The single subtests in a
<tt class="docutils literal">.placet</tt> file are not independent and it makes sense to exit
immediately at the first failure.</p>
<p>The support for doctests in <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> comes nearly for free, thanks to the
<a class="reference external" href="http://docs.python.org/library/shlex.html">shlex</a> module in the standard library, which is able to parse simple
languages as the ones you can implement with <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>. In particular,
thanks to <a class="reference external" href="http://docs.python.org/library/shlex.html">shlex</a>, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is able to recognize comments (the default
comment character is <tt class="docutils literal">#</tt>), escape sequences and more. Look at the
<a class="reference external" href="http://docs.python.org/library/shlex.html">shlex</a> documentation if you need to customize how the language is
interpreted. For more flexibility, it is even possible to pass to the
interpreter a custom split function with signature <tt class="docutils literal">split(line,
commentchar)</tt>.</p>
<p>In addition, I have implemented some support for line number
recognition, so that if a test fail you get the line number of the
failing command. This is especially useful if your tests are
stored in external files, even if they do not need to be in
a file: you can just pass to the <tt class="docutils literal">.doctest</tt> method a list of
strings corresponding to the lines of the file.</p>
<p>At the present <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> does not use any code from the doctest
module, but the situation may change in the future (it would be nice
if <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> could reuse doctests directives like ELLIPSIS).</p>
<p>It is straighforward to integrate your <tt class="docutils literal">.placet</tt> tests with standard
testing tools. For instance, you can integrate your doctests with <tt class="docutils literal">nose</tt>
or <tt class="docutils literal">py.test</tt> as follow:</p>
<pre class="literal-block">
import os, shlex, plac

def test_doct():
   &quot;&quot;&quot;
   Find all the doctests in the current directory and run them with the
   corresponding plac interpreter (the shebang rules!)
   &quot;&quot;&quot;
   placets = [f for f in os.listdir('.') if f.endswith('.placet')]
   for placet in placets:
       lines = list(open(placet))
       assert lines[0].startswith('#!'), 'Missing or incorrect shebang line!'
       firstline = lines[0][2:] # strip the shebang
       main = plac.import_main(*shlex.split(firstline))
       yield plac.Interpreter(main).doctest, lines[1:]
</pre>
<p>Here you should notice that usage of <tt class="docutils literal">plac.import_main</tt>, an utility
which is able to import the main function of the script specified in
the shebang line. You can use both the full path name of the
tool, or a relative path name. In this case the runner look at the
environment variable <tt class="docutils literal">PLACPATH</tt> and it searches
the plac tool in the directories specified there (<tt class="docutils literal">PLACPATH</tt> is just
a string containing directory names separated by colons). If the variable
<tt class="docutils literal">PLACPATH</tt> is not defined, it just looks in the current directory.
If the plac tool is not found, an <tt class="docutils literal">ImportError</tt> is raised.</p>
</div>
<div class="section" id="plac-batch-scripts">
<h2><a class="toc-backref" href="#id35">Plac batch scripts</a></h2>
<p>It is pretty easy to realize that an interactive interpreter can
also be used to run batch scripts: instead of reading the commands from
the console, it is enough to read the commands from a file.
<a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> interpreters provide an <tt class="docutils literal">.execute</tt> method to perform just that.</p>
<p>There is just a subtle point to notice: whereas in an interactive loop
one wants to manage all exceptions, a batch script should not continue in the
background in case of unexpected errors. The implementation of
<tt class="docutils literal">Interpreter.execute</tt> makes sure that any error raised by
<tt class="docutils literal">plac.call</tt> internally is re-raised.  In other words, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>
interpreters <em>wrap the errors, but does not eat them</em>: the errors are
always accessible and can be re-raised on demand.</p>
<p>The exception is the case of invalid commands, which are skipped.
Consider for instance the following batch file, which contains a
mispelled command (<tt class="docutils literal">.dl</tt> instead of <tt class="docutils literal">.del</tt>):</p>
<pre class="literal-block">
#!ishelve.py
.clear 
a=1 b=2
.show
.del a
.dl b
.show

</pre>
<p>If you execute the batch file, the interpreter will print a <tt class="docutils literal">.dl: not found</tt>
at the <tt class="docutils literal">.dl</tt> line and will continue:</p>
<pre class="literal-block">
$ python -c &quot;import plac, ishelve
plac.Interpreter(ishelve.main).execute(open('ishelve.plac'), verbose=True)&quot;
i&gt; .clear
cleared the shelve
i&gt; a=1 b=2
setting a=1
setting b=2
i&gt; .show
b=2
a=1
i&gt; .del a
deleted a
i&gt; .dl b
2
.dl: not found
i&gt; .show
b=2
</pre>
<p>The <tt class="docutils literal">verbose</tt> flag is there to show the lines which are being interpreted
(prefixed by <tt class="docutils literal">i&gt;</tt>). This is done on purpose, so that you can cut and paste
the output of the batch script and turn it into a <tt class="docutils literal">.placet</tt> test
(cool, isn't it?).</p>
</div>
<div class="section" id="implementing-subcommands">
<h2><a class="toc-backref" href="#id36">Implementing subcommands</a></h2>
<p>When I discussed the <tt class="docutils literal">ishelve</tt> implementation in the <a class="reference external" href="http://plac.googlecode.com/hg/doc/plac.html">basic
documentation</a>, I said that it looked like the poor man implementation
of an object system as a chain of elifs; I also said that <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> was
able to do much better than that.  Here I will substantiate my claim.</p>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is actually able to infer a set of subparsers from a
generic container of commands.  This is useful if you want to
implement <em>subcommands</em> (a familiar example of a command-line
application featuring subcommands is version control system).
Technically a container of commands is any object with a <tt class="docutils literal">.commands</tt> attribute
listing a set of functions or methods which are valid commands. A command
container may have initialization/finalization hooks (<tt class="docutils literal">__enter__/__exit__</tt>)
and dispatch hooks (<tt class="docutils literal">__missing__</tt>, invoked for invalid command names).
Moreover, only when using command containers <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is able to provide
automatic <em>autocompletion</em> of commands.</p>
<p>The shelve interface can be rewritten in an object-oriented way as follows:</p>
<pre class="literal-block">
# ishelve2.py
import shelve, os, plac

class ShelveInterface(object):
    &quot;A minimal interface over a shelve object.&quot;
    commands = 'set', 'show', 'showall', 'delete'
    &#64;plac.annotations(
        configfile=('path name of the shelve', 'option'))
    def __init__(self, configfile):
        self.configfile = configfile or '~/conf.shelve'
        self.fname = os.path.expanduser(self.configfile)
        self.__doc__ += '\nOperating on %s.\nUse help to see '\
            'the available commands.\n'  % self.fname
    def __enter__(self):
        self.sh = shelve.open(self.fname)
        return self
    def __exit__(self, etype, exc, tb):
        self.sh.close()
    def set(self, name, value):
        &quot;set name value&quot;
        yield 'setting %s=%s' % (name, value)
        self.sh[name] = value
    def show(self, *names):
        &quot;show given parameters&quot;
        for name in names:
            yield '%s = %s' % (name, self.sh[name]) # no error checking
    def showall(self):
        &quot;show all parameters&quot;
        for name in self.sh:
            yield '%s = %s' % (name, self.sh[name])
    def delete(self, name=None):
        &quot;delete given parameter (or everything)&quot;
        if name is None:
            yield 'deleting everything'
            self.sh.clear()
        else:
            yield 'deleting %s' % name
            del self.sh[name] # no error checking

if __name__ == '__main__':
    plac.Interpreter(plac.call(ShelveInterface)).interact()

</pre>
<p><tt class="docutils literal">plac.Interpreter</tt> objects wrap context manager objects
consistently.  In other words, if you wrap an object with
<tt class="docutils literal">__enter__</tt> and <tt class="docutils literal">__exit__</tt> methods, they are invoked in the right
order (<tt class="docutils literal">__enter__</tt> before the interpreter loop starts and
<tt class="docutils literal">__exit__</tt> after the interpreter loop ends, both in the regular and
in the exceptional case). In our example, the methods <tt class="docutils literal">__enter__</tt>
and <tt class="docutils literal">__exit__</tt> make sure the the shelve is opened and closed
correctly even in the case of exceptions. Notice that I have not
implemented any error checking in the <tt class="docutils literal">show</tt> and <tt class="docutils literal">delete</tt> methods
on purpose, to verify that <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> works correctly in the presence of
exceptions.</p>
<p>When working with command containers, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> automatically adds two
special commands to the set of provided commands: <tt class="docutils literal">help</tt>
and <tt class="docutils literal">.last_tb</tt>. The <tt class="docutils literal">help</tt> command is the easier to understand:
when invoked without arguments it displays the list of available commands
with the same formatting of the <a class="reference external" href="http://docs.python.org/library/cmd.html">cmd</a> module; when invoked with the name of
a command it displays the usage message for that command.
The <tt class="docutils literal">.last_tb</tt> command is useful when debugging: in case of errors,
it allows you to display the traceback of the last executed command.</p>
<p>Here is the usage message:</p>
<pre class="literal-block">
usage: ishelve2.py [-h] [-configfile CONFIGFILE]

A minimal interface over a shelve object.

optional arguments:
  -h, --help            show this help message and exit
  -configfile CONFIGFILE
                        path name of the shelve

</pre>
<p>Here is a session of usage on an Unix-like operating system:</p>
<pre class="literal-block">
$ python ishelve2.py -c ~/test.shelve
A minimal interface over a shelve object.
Operating on /home/micheles/test.shelve.
Use help to see the available commands.
i&gt; help

special commands
================
.last_tb

custom commands
===============
delete  set  show  showall

i&gt; delete
deleting everything
i&gt; set a pippo
setting a=pippo
i&gt; set b lippo
setting b=lippo
i&gt; showall
b = lippo
a = pippo
i&gt; show a b
a = pippo
b = lippo
i&gt; del a
deleting a
i&gt; showall
b = lippo
i&gt; delete a
deleting a
KeyError: 'a'
i&gt; .last_tb
 File &quot;/usr/local/lib/python2.6/dist-packages/plac-0.6.0-py2.6.egg/plac_ext.py&quot;, line 190, in _wrap
    for value in genobj:
  File &quot;./ishelve2.py&quot;, line 37, in delete
    del self.sh[name] # no error checking
  File &quot;/usr/lib/python2.6/shelve.py&quot;, line 136, in __delitem__
    del self.dict[key]
i&gt;
</pre>
<p>Notice that in interactive mode the traceback is hidden, unless
you pass the <tt class="docutils literal">verbose</tt> flag to the <tt class="docutils literal">Interpreter.interact</tt> method.</p>
<p>CHANGED IN VERSION 0.9: if you have an old version of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> the
<tt class="docutils literal">help</tt> command must be prefixed with a dot, i.e. you must write
<tt class="docutils literal">.help</tt>. The old behavior was more consistent in my opinion, since
it made it clear that the <tt class="docutils literal">help</tt> command was special and threated
differently from the regular commands.
Notice that if you implement a custom <tt class="docutils literal">help</tt> command in the commander class
the default help will not be added, as you would expect.</p>
<p>In version 0.9 an exception <tt class="docutils literal">`plac.Interpreter.Exit</tt> was added. Its
purpose it to make it easy to define commands to exit from the command
loop. Just define something like:</p>
<pre class="literal-block">
def quit(self):
   raise plac.Interpreter.Exit
</pre>
<p>and the interpreter will be closed properly when the <tt class="docutils literal">quit</tt> command
is entered.</p>
</div>
<div class="section" id="plac-interpreter-call">
<h2><a class="toc-backref" href="#id37">plac.Interpreter.call</a></h2>
<p>At the core of <tt class="docutils literal">plac</tt> there is the <tt class="docutils literal">call</tt> function which invokes
a callable with the list of arguments passed at the command-line
(<tt class="docutils literal">sys.argv[1:]</tt>). Thanks to <tt class="docutils literal">plac.call</tt> you can launch your module
by simply adding the lines:</p>
<pre class="literal-block">
if __name__ == '__main__':
    plac.call(main)
</pre>
<p>Everything works fine if <tt class="docutils literal">main</tt> is a simple callable performing some
action; however, in many cases, one has a <tt class="docutils literal">main</tt> &quot;function&quot; which
is a actually a factory returning a command container object. For
instance, in my second shelve example the main function is the class
<tt class="docutils literal">ShelveInterface</tt>, and the two lines needed to run the module are
a bit ugly:</p>
<pre class="literal-block">
if __name__ == '__main__':
   plac.Interpreter(plac.call(ShelveInterface)).interact()
</pre>
<p>Moreover, now the program runs, but only in interactive mode, i.e.
it is not possible to run it as a script. Instead, it would be nice
to be able to specify the command to execute on the command-line
and have the interpreter start, execute the command and finish
properly (I mean by calling <tt class="docutils literal">__enter__</tt> and <tt class="docutils literal">__exit__</tt>)
without needing user input. Then the script could be called from
a batch shell script working in the background.
In order to provide such functionality <tt class="docutils literal">plac.Interpreter</tt> provides
a classmethod named <tt class="docutils literal">.call</tt> which takes the factory, instantiates
it with the arguments read from the command line, wraps the resulting
container object as an interpreter and runs it with the rest arguments
found in the command line. Here is the code to turn the <tt class="docutils literal">ShelveInterface</tt>
into a script</p>
<pre class="literal-block">
# ishelve3.py
from ishelve2 import ShelveInterface

if __name__ == '__main__':
    import plac; plac.Interpreter.call(ShelveInterface)

## try the following:
# $ python ishelve3.py delete
# $ python ishelve3.py set a 1
# $ python ishelve3.py showall

</pre>
<p>and here are a few examples of usage:</p>
<pre class="literal-block">
$ python ishelve3.py help

special commands
================
.last_tb

custom commands
===============
delete  set  show  showall

$ python ishelve3.py set a 1
setting a=1
$ python ishelve3.py show a
a = 1
</pre>
<p>If you pass the <tt class="docutils literal"><span class="pre">-i</span></tt> flag in the command line, then the
script will enter in interactive mode and ask the user
for the commands to execute:</p>
<pre class="literal-block">
$ python ishelve3.py -i
A minimal interface over a shelve object.
Operating on /home/micheles/conf.shelve.
Use help to see the available commands.

i&gt;
</pre>
<p>In a sense, I have closed the circle: at the beginning of this
document I discussed how to turn a script into an interactive
application (the <tt class="docutils literal">shelve_interpreter.py</tt> example), whereas here I
have show how to turn an interactive application into a script.</p>
<p>The complete signature of <tt class="docutils literal">plac.Interpreter.call</tt> is the following:</p>
<pre class="literal-block">
call(factory, arglist=sys.argv[1:],
     commentchar='#', split=shlex.split,
     stdin=sys.stdin, prompt='i&gt; ', verbose=False)
</pre>
<p>The factory must have a fixed number of positional arguments (no
default arguments, no varargs, no kwargs), otherwise a <tt class="docutils literal">TypeError</tt>
is raised: the reason is that we want to be able to distinguish the
command-line arguments needed to instantiate the factory from the rest
arguments that must be sent to the corresponding interpreter object.
It is also possible to specify a list of arguments different from
<tt class="docutils literal">sys.argv[1:]</tt> (useful in tests), the character to be recognized as
a comment, the splitting function, the input source and the prompt to
use while in interactive mode, and a verbose flag.</p>
</div>
<div class="section" id="readline-support">
<h2><a class="toc-backref" href="#id38">Readline support</a></h2>
<p>Starting from release 0.6 <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> offers full readline support. That
means that if your Python was compiled with readline support you get
autocompletion and persistent command history for free.  By default
all commands are autocomplete in a case sensitive way.  If you want to
add new words to the autocompletion set, or you want to change the
location of the <tt class="docutils literal">.history</tt> file, or to change the case sensitivity,
the way to go is to pass a <tt class="docutils literal">plac.ReadlineInput</tt> object to the
interpreter.  Here is an example, assuming you want to build a
database interface understanding SQL commands:</p>
<pre class="literal-block">
import os, plac
from sqlalchemy.ext.sqlsoup import SqlSoup

SQLKEYWORDS = set(['help', 'select', 'from', 
                   'inner', 'join', 'outer', 'left', 'right']
                  ) # and many others
DBTABLES = set(['table1', 'table2']) # you can read them from the db schema

COMPLETIONS = SQLKEYWORDS | DBTABLES

class SqlInterface(object):
    commands = ['SELECT']
    def __init__(self, dsn):
        self.soup = SqlSoup(dsn)
    def SELECT(self, argstring):
        sql = 'SELECT ' + argstring
        for row in self.soup.bind.execute(sql):
            yield str(row) # the formatting can be much improved

rl_input = plac.ReadlineInput(
    COMPLETIONS, histfile=os.path.expanduser('~/.sql_interface.history'), 
    case_sensitive=False)

def split_on_first_space(line, commentchar):
    return line.strip().split(' ', 1) # ignoring comments
    
if __name__ == '__main__':
    plac.Interpreter.call(SqlInterface, split=split_on_first_space,
                          stdin=rl_input, prompt='sql&gt; ')

</pre>
<p>Here is an example of usage:</p>
<pre class="literal-block">
$ python sql_interface.py &lt;some dsn&gt;
sql&gt; SELECT a.* FROM TABLE1 AS a INNER JOIN TABLE2 AS b ON a.id = b.id
...
</pre>
<p>You can check that entering just <tt class="docutils literal">sel</tt> and pressing TAB the readline library
completes the <tt class="docutils literal">SELECT</tt> keyword for you and makes it upper case; idem for
<tt class="docutils literal">FROM</tt>, <tt class="docutils literal">INNER</tt>, <tt class="docutils literal">JOIN</tt> and even for the names of the tables. An
obvious improvement is to read the names of the tables by introspecting
the database: actually you can even read the names of the views and of
the columns, and have full autocompletion. All the entered commands
and recorded and saved in the file <tt class="docutils literal"><span class="pre">~/.sql_interface.history</span></tt> when
exiting from the command-line interface.</p>
<p>If the readline library is not available, my suggestion is to use the
<a class="reference external" href="http://freshmeat.net/projects/rlwrap/">rlwrap</a> tool which provides similar features, at least on Unix-like
platforms. <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> should also work fine on Windows with the <a class="reference external" href="http://ipython.scipy.org/moin/PyReadline/Intro">pyreadline</a>
library (I do not use Windows, so this part is very little tested: I
tried it only once and it worked, but your mileage may vary).
For people worried about licenses, I will notice that <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> uses the
readline library only if available, it does not include it and it does
not rely on it in any fundamental way, so that the <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> licence does
not need to be the GPL (actually it is a BSD
do-whatever-you-want-with-it licence).</p>
<p>The interactive mode of <tt class="docutils literal">plac</tt> can be used as a replacement of the
<a class="reference external" href="http://docs.python.org/library/cmd.html">cmd</a> module in the standard library. It is actually better than <a class="reference external" href="http://docs.python.org/library/cmd.html">cmd</a>:
for instance, the <tt class="docutils literal">help</tt> command is more powerful, since it
provides information about the arguments accepted by the given command:</p>
<pre class="literal-block">
i&gt; help set
usage:  set name value

set name value

positional arguments:
  name
  value

i&gt; help delete
usage:  delete [name]

delete given parameter (or everything)

positional arguments:
  name        [None]

i&gt; help show
usage:  show [names [names ...]]

show given parameters

positional arguments:
  names
</pre>
<p>As you can imagine, the help message is provided by the underlying <a class="reference external" href="http://argparse.googlecode.com">argparse</a>
subparser: there is a subparser for each command. <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> commands accept
options, flags, varargs, keyword arguments, arguments with defaults,
arguments with a fixed number of choices, type conversion and all the
features provided of <a class="reference external" href="http://argparse.googlecode.com">argparse</a> .</p>
<p>Moreover at the moment <tt class="docutils literal">plac</tt> also understands command abbreviations.
However, this feature may disappear in
future releases. It was meaningful in the past, when <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> did not support
readline.</p>
<p>Notice that if an abbreviation is ambiguous, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> warns you:</p>
<pre class="literal-block">
i&gt; sh
NameError: Ambiguous command 'sh': matching ['showall', 'show']
</pre>
</div>
<div class="section" id="the-plac-runner">
<h2><a class="toc-backref" href="#id39">The plac runner</a></h2>
<p>The distribution of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> includes a runner script named <tt class="docutils literal">plac_runner.py</tt>,
which will be installed in a suitable directory in your system by <a class="reference external" href="http://docs.python.org/distutils/">distutils</a>
(say in <tt class="docutils literal">\usr\local\bin\plac_runner.py</tt> in a Unix-like operative system).
The runner provides many facilities to run <tt class="docutils literal">.plac</tt> scripts and
<tt class="docutils literal">.placet</tt> files, as well as Python modules containg a <tt class="docutils literal">main</tt>
object, which can be a function, a command container object or
even a command container class.</p>
<p>For instance, suppose you want to execute a script containing commands
defined in the <tt class="docutils literal">ishelve2</tt> module like the following one:</p>
<pre class="literal-block">
#!ishelve2.py:ShelveInterface -c ~/conf.shelve
set a 1
del a
del a # intentional error

</pre>
<p>The first line of the <tt class="docutils literal">.plac</tt> script contains the name of the
python module containing the plac interpreter and the arguments
which must be passed to its main function in order to be able
to instantiate an interpreter object. In this case I appended
<tt class="docutils literal">:ShelveInterface</tt> to the name of the module to specify the
object that must be imported: if not specified, by default the
object named 'main' is imported.
The other lines contains commands.
You can run the script as follows:</p>
<pre class="literal-block">
$ plac_runner.py --batch ishelve2.plac
setting a=1
deleting a
Traceback (most recent call last):
  ...
_bsddb.DBNotFoundError: (-30988, 'DB_NOTFOUND: No matching key/data pair found')
</pre>
<p>The last command intentionally contained an error, to show that the
plac runner does not eat the traceback.</p>
<p>The runner can also be used to run Python modules in interactive
mode and non-interactive mode. If you put this alias in your bashrc</p>
<blockquote>
<tt class="docutils literal">alias <span class="pre">plac=&quot;plac_runner.py&quot;</span></tt></blockquote>
<p>(or you define a suitable <tt class="docutils literal">plac.bat</tt> script in Windows) you can
run the <tt class="docutils literal">ishelve2.py</tt> script in interactive mode as
follows:</p>
<pre class="literal-block">
$ plac -i ishelve2.py:ShelveInterface
A minimal interface over a shelve object.
Operating on /home/micheles/conf.shelve.
.help to see the available commands.

i&gt; del
deleting everything
i&gt; set a 1
setting a=1
i&gt; set b 2
setting b=2
i&gt; show b
b = 2
</pre>
<p>Now you can cut and paste the interactive session an turns into into
a <tt class="docutils literal">.placet</tt> file like the following:</p>
<pre class="literal-block">
#!ishelve2.py:ShelveInterface -configfile=~/test.shelve
# an example of .placet file for the ShelveInterface
i&gt; del
deleting everything
i&gt; set a 1
setting a=1
i&gt; set b 2
setting b=2
i&gt; show a
a = 1

</pre>
<p>Notice that the first line specifies a test database
<tt class="docutils literal">~/test.shelve</tt>, to avoid clobbering your default shelve. If you
mispell the arguments in the first line plac will give you an
<a class="reference external" href="http://argparse.googlecode.com">argparse</a> error message (just try).</p>
<p>You can run placets following the shebang convention directly with
the plac runner:</p>
<pre class="literal-block">
$ plac --test ishelve2.placet
run 1 plac test(s)
</pre>
<p>If you want to see the output of the tests, pass the <tt class="docutils literal"><span class="pre">-v/--verbose</span></tt> flag.
Notice that he runner ignore the extension, so you can actually use any
extension your like, but <em>it relies on the first line of the file to invoke
the corresponding plac tool with the given arguments</em>.</p>
<p>The plac runner does not provide any test discovery facility,
but you can use standard Unix tools to help. For instance, you can
run all the <tt class="docutils literal">.placet</tt> files into a directory and its subdirectories
as follows:</p>
<pre class="literal-block">
$ find . -name \*.placet | xargs plac_runner.py -t
</pre>
<p>The plac runner expects the main function of your script to
return a plac tool, i.e. a function or an object with a <tt class="docutils literal">.commands</tt>
attribute. It this is not the case the runner gracefully exits.</p>
<p>It also works in non-interactive mode, if you call it as</p>
<blockquote>
<tt class="docutils literal">$ plac module.py args ...</tt></blockquote>
<p>Here is an example:</p>
<pre class="literal-block">
$ plac ishelve.py a=1
setting a=1
$ plac ishelve.py .show
a=1
</pre>
<p>Notice that in non-interactive mode the runner just invokes <tt class="docutils literal">plac.call</tt>
on the <tt class="docutils literal">main</tt> object of the Python module.</p>
</div>
<div class="section" id="a-non-class-based-example">
<h2><a class="toc-backref" href="#id40">A non class-based example</a></h2>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> does not force you to use classes to define command containers.
Even a simple function can be a valid command container, it is
enough to add to it a <tt class="docutils literal">.commands</tt> attribute and possibly
<tt class="docutils literal">__enter__</tt> and/or <tt class="docutils literal">__exit__</tt> attributes.</p>
<p>In particular, a Python module is a perfect container of commands. As an
example, consider the following module implementing a fake Version
Control System:</p>
<pre class="literal-block">
&quot;A Fake Version Control System&quot;

import plac # this implementation also works with Python 2.4

commands = 'checkout', 'commit', 'status'

&#64;plac.annotations(url='url of the source code')
def checkout(url):
    &quot;A fake checkout command&quot;
    return ('checkout ', url)

&#64;plac.annotations(message=('commit message', 'option'))
def commit(message):
    &quot;A fake commit command&quot;
    return ('commit ', message)

&#64;plac.annotations(quiet=('summary information', 'flag', 'q'))
def status(quiet):
    &quot;A fake status command&quot;
    return ('status ', quiet)

def __missing__(name):
    return ('Command %r does not exist' % name,)

def __exit__(etype, exc, tb):
    &quot;Will be called automatically at the end of the intepreter loop&quot;
    if etype in (None, GeneratorExit): # success
        print('ok')

main = __import__(__name__) # the module imports itself!

if __name__ == '__main__':
    import plac
    for out in plac.call(main): print(out)

</pre>
<p>Notice that I have defined both an <tt class="docutils literal">__exit__</tt> hook and a <tt class="docutils literal">__missing__</tt>
hook, invoked for non-existing commands.
The real trick here is the line <tt class="docutils literal">main = __import__(__name__)</tt>, which
define <tt class="docutils literal">main</tt> to be an alias for the current module.</p>
<p>The <tt class="docutils literal">vcs</tt> module can be run through the plac runner
(try <tt class="docutils literal">plac vcs.py <span class="pre">-h</span></tt>):</p>
<pre class="literal-block">
usage: plac_runner.py vcs.py [-h] {status,commit,checkout} ...

A Fake Version Control System

optional arguments:
  -h, --help            show this help message and exit

subcommands:
  {status,commit,checkout}
    checkout            A fake checkout command
    commit              A fake commit command
    status              A fake status command

</pre>
<p>You can get help for the subcommands by inserting an <tt class="docutils literal"><span class="pre">-h</span></tt> close the
name of the command:</p>
<pre class="literal-block">
$ plac vcs.py status -h
usage: plac_runner.py vcs.py status [-h] [-q]

A fake status command

optional arguments:
  -h, --help   show this help message and exit
  -q, --quiet  summary information
</pre>
<p>Notice how the docstring of the command is automatically shown in
usage message, as well as the documentation for the sub flag <tt class="docutils literal"><span class="pre">-q</span></tt>.</p>
<p>Here is an example of a non-interactive session:</p>
<pre class="literal-block">
$ plac vcs.py check url
checkout
url
$ plac vcs.py st -q
status
True
$ plac vcs.py co
commit
None
</pre>
<p>and here is an interactive session:</p>
<pre class="literal-block">
$ plac -i vcs.py
usage: plac_runner.py vcs.py [-h] {status,commit,checkout} ...
i&gt; check url
checkout
url
i&gt; st -q
status
True
i&gt; co
commit
None
i&gt; sto
Command 'sto' does not exist
i&gt; [CTRL-D]
ok
</pre>
<p>Notice the invocation of the <tt class="docutils literal">__missing__</tt> hook for non-existing commands.
Notice also that the <tt class="docutils literal">__exit__</tt> hook gets called only in interactive
mode.</p>
<p>If the commands are completely independent, a module is a good fit for
a method container. In other situations, it is best to use a custom
class.</p>
</div>
<div class="section" id="writing-your-own-plac-runner">
<h2><a class="toc-backref" href="#id41">Writing your own plac runner</a></h2>
<p>The runner included in the <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> distribution is intentionally kept
small (around 50 lines of code) so that you can study it and write
your own runner if want to. If you need to go to such level
of detail, you should know that the most important method of
the <tt class="docutils literal">Interpreter</tt> class is the <tt class="docutils literal">.send</tt> method, which takes
strings in input and returns a four-tuple with attributes
<tt class="docutils literal">.str</tt>, <tt class="docutils literal">.etype</tt>, <tt class="docutils literal">.exc</tt> and <tt class="docutils literal">.tb</tt>:</p>
<ul class="simple">
<li><tt class="docutils literal">.str</tt> is the output of the command, if successful (a string);</li>
<li><tt class="docutils literal">.etype</tt> is the class of the exception, if the command fail;</li>
<li><tt class="docutils literal">.exc</tt> is the exception instance;</li>
<li><tt class="docutils literal">.tb</tt> is the traceback.</li>
</ul>
<p>Moreover the <tt class="docutils literal">__str__</tt> representation of the output object is redefined
to return the output string if the command was successful or the error
message if the command failed (actually it returns the error message
preceded by the name of the exception class).</p>
<p>For instance, if you send a mispelled option to
the interpreter a <tt class="docutils literal">SystemExit</tt> will be trapped:</p>
<pre class="doctest-block">
&gt;&gt;&gt; import plac
&gt;&gt;&gt; from ishelve import ishelve
&gt;&gt;&gt; with plac.Interpreter(ishelve) as i:
...     print(i.send('.cler'))
...
SystemExit: unrecognized arguments: .cler
</pre>
<p>It is important to invoke the <tt class="docutils literal">.send</tt> method inside the context manager,
otherwise you will get a <tt class="docutils literal">RuntimeError</tt>.</p>
<p>For instance, suppose you want to implement a graphical runner for a
plac-based interpreter with two text widgets: one to enter the commands
and one to display the results. Suppose you want to display the errors
with tracebacks in red. You will need to code something like that
(pseudocode follows):</p>
<pre class="literal-block">
input_widget = WidgetReadingInput()
output_widget = WidgetDisplayingOutput()

def send(interpreter, line):
    out = interpreter.send(line)
    if out.tb: # there was an error
        output_widget.display(out.tb, color='red')
    else:
        output_widget.display(out.str)

main = plac.import_main(tool_path) # get the main object

with plac.Interpreter(main) as i:
   def callback(event):
      if event.user_pressed_ENTER():
           send(i, input_widget.last_line)
   input_widget.addcallback(callback)
   gui_mainloop.start()
</pre>
<p>You can adapt the pseudocode to your GUI toolkit of choice and you can
also change the file associations in such a way that clicking on a
plac tool file the graphical user interface starts.</p>
<p>An example of GUI program built on top of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is given later on, in the
paragraph <em>Managing the output of concurrent commands</em> (using Tkinter
for simplicity and portability).</p>
<p>There is a final <em>caveat</em>: since the plac interpreter loop is
implemented via extended generators, plac interpreters are single threaded: you
will get an error if you <tt class="docutils literal">.send</tt> commands from separated threads.
You can circumvent the problem by using a queue. If EXIT is a sentinel
value to signal exiting from the interpreter look, you can write code
like this:</p>
<pre class="literal-block">
with interpreter:
    for input_value in iter(input_queue.get, EXIT):
        output_queue.put(interpreter.send(input_value))
</pre>
<p>The same trick also work for processes; you could run the interpreter
loop in a separate process and send commands to it via the Queue
class provided by the <a class="reference external" href="http://docs.python.org/library/multiprocessing.html">multiprocessing</a> module.</p>
</div>
<div class="section" id="long-running-commands">
<h2><a class="toc-backref" href="#id42">Long running commands</a></h2>
<p>As we saw, by default a <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> interpreter blocks until
the command terminates. This is an issue, in the sense that it makes
the interactive experience quite painful for long running commands. An
example is better than a thousand words, so consider the following
fake importer:</p>
<pre class="literal-block">
import time
import plac

class FakeImporter(object):
    &quot;A fake importer with an import_file command&quot;
    commands = ['import_file']
    def __init__(self, dsn):
        self.dsn = dsn
    def import_file(self, fname):
        &quot;Import a file into the database&quot;
        try:
            for n in range(10000):
                time.sleep(.01)
                if n % 100 == 99:
                    yield 'Imported %d lines' % (n+1)
        finally:
            print('closing the file')

if __name__ == '__main__':
    plac.Interpreter.call(FakeImporter)

</pre>
<p>If you run the <tt class="docutils literal">import_file</tt> command, you will have to wait for 200 seconds
before entering a new command:</p>
<pre class="literal-block">
$ python importer1.py dsn -i
A fake importer with an import_file command
i&gt; import_file file1
... &lt;wait 3+ minutes&gt;
Imported 100 lines
Imported 200 lines
Imported 300 lines
...
Imported 10000 lines
closing the file
</pre>
<p>Being unable to enter any other command is quite annoying: in such situation one
would like to run the long running commands in the background, to keep
the interface responsive. <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> provides two ways to reach this goal: threads
and processes.</p>
</div>
<div class="section" id="threaded-commands">
<h2><a class="toc-backref" href="#id43">Threaded commands</a></h2>
<p>The most familiar way to execute a task in the background (even if not
necessarily the best way) is to run it into a separated thread. In our
example it is sufficient to replace the line</p>
<blockquote>
<tt class="docutils literal">commands = ['import_file']</tt></blockquote>
<p>with</p>
<blockquote>
<tt class="docutils literal">thcommands = ['import_file']</tt></blockquote>
<p>to tell to the <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> interpreter that the command <tt class="docutils literal">import_file</tt> should be
run into a separated thread. Here is an example session:</p>
<pre class="literal-block">
i&gt; import_file file1
&lt;ThreadedTask 1 [import_file file1] RUNNING&gt;
</pre>
<p>The import task started in a separated thread. You can see the
progress of the task by using the special command <tt class="docutils literal">.output</tt>:</p>
<pre class="literal-block">
i&gt; .output 1
&lt;ThreadedTask 1 [import_file file1] RUNNING&gt;
Imported 100 lines
Imported 200 lines
</pre>
<p>If you look after a while, you will get more lines of output:</p>
<pre class="literal-block">
i&gt; .output 1
&lt;ThreadedTask 1 [import_file file1] RUNNING&gt;
Imported 100 lines
Imported 200 lines
Imported 300 lines
Imported 400 lines
</pre>
<p>If you look after a time long enough, the task will be finished:</p>
<pre class="literal-block">
i&gt; .output 1
&lt;ThreadedTask 1 [import_file file1] FINISHED&gt;
</pre>
<p>It is possible to store the output of a task into a file, to be read
later (this is useful for tasks with a large output):</p>
<pre class="literal-block">
i&gt; .output 1 /tmp/out.txt
saved output of 1 into /tmp/out.txt
</pre>
<p>You can even skip the number argument: then <tt class="docutils literal">.output</tt> will the return
the output of the last launched command (the special commands like .output
do not count).</p>
<p>You can launch many tasks one after the other:</p>
<pre class="literal-block">
i&gt; import_file file2
&lt;ThreadedTask 5 [import_file file2] RUNNING&gt;
i&gt; import_file file3
&lt;ThreadedTask 6 [import_file file3] RUNNING&gt;
</pre>
<p>The <tt class="docutils literal">.list</tt> command displays all the running tasks:</p>
<pre class="literal-block">
i&gt; .list
&lt;ThreadedTask 5 [import_file file2] RUNNING&gt;
&lt;ThreadedTask 6 [import_file file3] RUNNING&gt;
</pre>
<p>It is even possible to kill a task:</p>
<pre class="literal-block">
i&gt; .kill 5
&lt;ThreadedTask 5 [import_file file2] TOBEKILLED&gt;
# wait a bit ...
closing the file
i&gt; .output 5
&lt;ThreadedTask 5 [import_file file2] KILLED&gt;
</pre>
<p>You should notice that since at the Python level it is impossible to kill
a thread, the <tt class="docutils literal">.kill</tt> commands works by setting the status of the task to
<tt class="docutils literal">TOBEKILLED</tt>. Internally the generator corresponding to the command
is executed in the thread and the status is checked at each iteration:
when the status become <tt class="docutils literal">TOBEKILLED</tt> a <tt class="docutils literal">GeneratorExit</tt> exception is
raised and the thread terminates (softly, so that the <tt class="docutils literal">finally</tt> clause
is honored). In our example the generator is yielding
back control once every 100 iterations, i.e. every two seconds (not much).
In order to get a responsive interface it is a good idea to yield more
often, for instance every 10 iterations (i.e. 5 times per second),
as in the following code:</p>
<pre class="literal-block">
import time
import plac

class FakeImporter(object):
    &quot;A fake importer with an import_file command&quot;
    thcommands = ['import_file']
    def __init__(self, dsn):
        self.dsn = dsn
    def import_file(self, fname):
        &quot;Import a file into the database&quot;
        try:
            for n in range(10000):
                time.sleep(.02)
                if n % 100 == 99: # every two seconds
                    yield 'Imported %d lines' % (n+1)
                if n % 10 == 9: # every 0.2 seconds
                    yield # go back and check the TOBEKILLED status
        finally:
            print('closing the file')

if __name__ == '__main__':
    plac.Interpreter.call(FakeImporter)

</pre>
</div>
<div class="section" id="running-commands-as-external-processes">
<h2><a class="toc-backref" href="#id44">Running commands as external processes</a></h2>
<p>Threads are not loved much in the Python world and actually most people
prefer to use processes instead. For this reason <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> provides the
option to execute long running commands as external processes. Unfortunately
the current implementation only works in Unix-like operating systems
(including Mac OS X) because it relies on fork via the <a class="reference external" href="http://docs.python.org/library/multiprocessing.html">multiprocessing</a>
module.</p>
<p>In our example, to enable the feature it is sufficient to replace the line</p>
<blockquote>
<tt class="docutils literal">thcommands = ['import_file']</tt></blockquote>
<p>with</p>
<blockquote>
<tt class="docutils literal">mpcommands = ['import_file']</tt>.</blockquote>
<p>The user experience is exactly the same as with threads and you will not see any
difference at the user interface level:</p>
<pre class="literal-block">
i&gt; import_file file3
&lt;MPTask 1 [import_file file3] SUBMITTED&gt;
i&gt; .kill 1
&lt;MPTask 1 [import_file file3] RUNNING&gt;
closing the file
i&gt; .output 1
&lt;MPTask 1 [import_file file3] KILLED&gt;
Imported 100 lines
Imported 200 lines
i&gt;
</pre>
<p>Still, using processes is quite different than using threads: in
particular, when using processes you can only yield pickleable values
and you cannot re-raise an exception first raised in a different
process, because traceback objects are not pickleable. Moreover,
you cannot rely on automatic sharing of your objects.</p>
<p>On the plus side, when using processes you do not need to worry about
killing a command: they are killed immediately using a SIGTERM signal,
and there is not a <tt class="docutils literal">TOBEKILLED</tt> mechanism. Moreover, the killing is
guaranteed to be soft: internally a command receiving a SIGTERM raises
a <tt class="docutils literal">TerminatedProcess</tt> exception which is trapped in the generator
loop, so that the command is closed properly.</p>
<p>Using processes allows to take full advantage of multicore machines
and it is safer than using threads, so it is the recommended approach
unless you are working on Windows.</p>
</div>
<div class="section" id="managing-the-output-of-concurrent-commands">
<h2><a class="toc-backref" href="#id45">Managing the output of concurrent commands</a></h2>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> acts as a command-line task launcher and can be used as the base
to build a GUI-based task launcher and task monitor. To this aim the
interpreter class provides a <tt class="docutils literal">.submit</tt> method which returns a task
object and a <tt class="docutils literal">.tasks</tt> method returning the list of all the tasks
submitted to the interpreter.  The <tt class="docutils literal">submit</tt> method does not start the task
and thus it is nonblocking.
Each task has an <tt class="docutils literal">.outlist</tt> attribute which is a list
storing the value yielded by the generator underlying the task (the
<tt class="docutils literal">None</tt> values are skipped though): the <tt class="docutils literal">.outlist</tt> grows as the
task runs and more values are yielded. Accessing the <tt class="docutils literal">.outlist</tt> is
nonblocking and can be done freely.
Finally there is a <tt class="docutils literal">.result</tt>
property which waits for the task to finish and returns the last yielded
value or raises an exception. The code below provides an example of
how you could implement a GUI over the importer example:</p>
<pre class="literal-block">
from __future__ import with_statement
from Tkinter import *
from importer3 import FakeImporter

def taskwidget(root, task, tick=500):
    &quot;A Label widget showing the output of a task every 500 ms&quot;
    sv = StringVar(root)
    lb = Label(root, textvariable=sv)
    def show_outlist():
        try:
            out = task.outlist[-1]
        except IndexError: # no output yet
            out = ''
        sv.set('%s %s' % (task, out))
        root.after(tick, show_outlist)
    root.after(0, show_outlist)
    return lb

def monitor(tasks):
    root = Tk()
    for task in tasks:
        task.run()
        taskwidget(root, task).pack()
    root.mainloop()

if __name__ == '__main__':
    import plac
    with plac.Interpreter(plac.call(FakeImporter)) as i:
        tasks = [i.submit('import_file f1'), i.submit('import_file f2')]
        monitor(tasks)

</pre>
</div>
</div>
<div class="section" id="experimental-features">
<h1><a class="toc-backref" href="#id46">Experimental features</a></h1>
<p>The distribution of <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> includes a few experimental features which I am
not committed to fully support and that may go away in future versions.
They are included as examples of things that you may build on top of
<a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>: the aim is to give you ideas. Some of the experimental features
might grow to become external projects build on <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a>.</p>
<div class="section" id="parallel-computing-with-plac">
<h2><a class="toc-backref" href="#id47">Parallel computing with plac</a></h2>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> is certainly not intended as a tool for parallel computing, but
still you can use it to launch a set of commands and to collect the
results, similarly to the MapReduce pattern popularized by
Google.  In order to give an example, I will consider the &quot;Hello
World&quot; of parallel computing, i.e. the computation of pi with
independent processes.  There is a huge number of algorithms to
compute pi; here I will describe a trivial one chosen for simplicity,
not per efficienty. The trick is to consider the first quadrant of a
circle with radius 1 and to extract a number of points <tt class="docutils literal">(x, y)</tt> with
<tt class="docutils literal">x</tt> and <tt class="docutils literal">y</tt> random variables in the interval <tt class="docutils literal">[0,1]</tt>. The
probability of extracting a number inside the quadrant (i.e. with
<tt class="docutils literal">x^2 + y^2 &lt; 1</tt>) is proportional to the area of the quadrant
(i.e. <tt class="docutils literal">pi/4</tt>). The value of <tt class="docutils literal">pi</tt> therefore can be extracted by
multiplying by 4 the ratio between the number of points in the
quadrant versus the total number of points <tt class="docutils literal">N</tt>, for <tt class="docutils literal">N</tt> large:</p>
<pre class="literal-block">
def calc_pi(N):
    inside = 0
    for j in xrange(N):
        x, y = random(), random()
        if x*x + y*y &lt; 1:
            inside += 1
    return (4.0 * inside) / N
</pre>
<p>The algorithm is trivially parallelizable: if you have n CPUs, you can
compute pi n times with N/n iterations, sum the results and divide the total
by n. I have a Macbook with two cores, therefore I would expect a speedup
factor of 2 with respect to a sequential computation. Moreover, I would
expect a threaded computation to be even slower than a sequential
computation, due to the GIL and the scheduling overhead.</p>
<p>Here is a script implementing the algorithm and working in three different
modes (parallel mode, threaded mode and sequential mode) depending on a
<tt class="docutils literal">mode</tt> option:</p>
<pre class="literal-block">
from __future__ import with_statement
from random import random
import multiprocessing
import plac

class PiCalculator(object):
    &quot;&quot;&quot;Compute pi in parallel with threads or processes&quot;&quot;&quot;
    
    &#64;plac.annotations(
        npoints=('number of integration points', 'positional', None, int),
        mode=('sequential|parallel|threaded', 'option', 'm', str, 'SPT'))
    def __init__(self, npoints, mode='S'):
        self.npoints = npoints
        if mode == 'P':
            self.mpcommands = ['calc_pi']
        elif mode == 'T':
            self.thcommands = ['calc_pi']
        elif mode == 'S':
            self.commands = ['calc_pi']
        self.n_cpu = multiprocessing.cpu_count()
    
    def submit_tasks(self):
        self.i = plac.Interpreter(self).__enter__()            
        return [self.i.submit('calc_pi %d' % (self.npoints / self.n_cpu))
                for _ in range(self.n_cpu)]

    def close(self):
        self.i.close()

    &#64;plac.annotations(
        npoints=('npoints', 'positional', None, int))
    def calc_pi(self, npoints):
        counts = 0
        for j in range(npoints):
            n, r = divmod(j, 1000000)
            if r == 0:
                yield '%dM iterations' % n
            x, y = random(), random()
            if x*x + y*y &lt; 1:
                counts += 1
        yield (4.0 * counts)/npoints

    def run(self):
        tasks = self.i.tasks()
        for t in tasks:
            t.run()
        try:
            total = 0
            for task in tasks:
                total += task.result
        except: # the task was killed
            print(tasks)
            return
        return total / self.n_cpu

if __name__ == '__main__':
    pc = plac.call(PiCalculator)
    pc.submit_tasks()
    try:
        import time; t0 = time.time()
        print('%f in %f seconds ' % (pc.run(), time.time() - t0))
    finally:
        pc.close()

</pre>
<p>Notice the <tt class="docutils literal">submit_tasks</tt> method, which instantiates and initializes a
<tt class="docutils literal">plac.Interpreter</tt> object and submits a number of commands corresponding
to the number of available CPUs. The <tt class="docutils literal">calc_pi</tt> command yield a log
message every million of interactions, just to monitor the progress of
the computation. The <tt class="docutils literal">run</tt> method starts all the submitted commands
in parallel and sums the results. It returns the average value of <tt class="docutils literal">pi</tt>
after the slowest CPU has finished its job (if the CPUs are equal and
equally busy they should finish more or less at the same time).</p>
<p>Here are the results on my old Macbook with Ubuntu 10.04 and Python 2.6,
for 10 million of iterations:</p>
<pre class="literal-block">
$ python picalculator.py -mP 10000000 # two processes
3.141904 in 5.744545 seconds
$ python picalculator.py -mT 10000000 # two threads
3.141272 in 13.875645 seconds
$ python picalculator.py -mS 10000000 # sequential
3.141586 in 11.353841 seconds
</pre>
<p>As you see using processes one gets a 2x speedup indeed, where the threaded
mode is some 20% slower than the sequential mode.</p>
<p>Since the pattern submit a bunch of tasks, starts them and collect the
results is so common, <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> provides an utility function
<tt class="docutils literal">runp(genseq, <span class="pre">mode='p')</span></tt> to start
a bunch a generators and return a list of results. By default
<tt class="docutils literal">runp</tt> use processes, but you can use threads by passing <tt class="docutils literal"><span class="pre">mode='t'</span></tt>.
With <tt class="docutils literal">runp</tt> the parallel pi calculation becomes a one-liner:</p>
<pre class="literal-block">
sum(task.result for task in plac.runp(calc_pi(N) for i in range(ncpus)))/ncpus
</pre>
<p>The file <tt class="docutils literal">test_runp</tt> in the <tt class="docutils literal">doc</tt> directory of the plac distribution
shows another example of usage. Notice that if one of the tasks fails
for some reason, instead of the result you will get the exception object.</p>
</div>
<div class="section" id="monitor-support">
<h2><a class="toc-backref" href="#id48">Monitor support</a></h2>
<p><a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> provides experimental support for monitoring the output of
concurrent commands, at least for platforms where multiprocessing is
fully supported. You can define your own monitor class, simply by
inheriting from <tt class="docutils literal">plac.Monitor</tt> and by overriding the methods
<tt class="docutils literal">add_listener(self, no)</tt>, <tt class="docutils literal">del_listener(self, taskno)</tt>,
<tt class="docutils literal">notify_listener(self, taskno, msg)</tt>, <tt class="docutils literal">read_queue(self)</tt>
<tt class="docutils literal">start(self)</tt> and <tt class="docutils literal">stop(self)</tt>.  Then, you can a monitor object to
any <tt class="docutils literal">plac.Interpreter</tt> object by simply calling the <tt class="docutils literal">add_monitor</tt>
method. For convenience, <tt class="docutils literal">plac</tt> comes with a very simple
<tt class="docutils literal">TkMonitor</tt> based on Tkinter (I chose Tkinter because it is easy to
use and it is in the standard library, but you can use any GUI): you
can just look at how the <tt class="docutils literal">TkMonitor</tt> is implemented in
<tt class="docutils literal">plac_tk.py</tt> and adapt it. Here is an example of usage of the
<tt class="docutils literal">TkMonitor</tt>:</p>
<pre class="literal-block">
from __future__ import with_statement
import plac

class Hello(object):
    mpcommands = ['hello', 'quit']
    def hello(self):
        yield 'hello'
    def quit(self):
        raise plac.Interpreter.Exit

if __name__ == '__main__':
    i = plac.Interpreter(Hello())
    i.add_monitor(plac.TkMonitor('tkmon'))
    i.interact()

    

</pre>
<p>Try to give the <tt class="docutils literal">hello</tt> command from the interactive interpreter:
each time a new text widget will be added displaying the output
of the command. Notice that if <tt class="docutils literal">Tkinter</tt> is not installed correctly
on your system the <tt class="docutils literal">TkMonitor</tt> class will not be available.</p>
</div>
<div class="section" id="the-plac-server">
<h2><a class="toc-backref" href="#id49">The plac server</a></h2>
<p>A command-line oriented interface can be easily converted into a
socket-based interface. Starting from release 0.7 plac features a
builtin server which is able to accept commands from multiple clients
and to execute them. The server works by instantiating a separate
interpreter for each client, so that if a client interpreter dies for
any reason the other interpreters keep working.  To avoid external
dependencies the server is based on the <tt class="docutils literal">asynchat</tt> module in the
standard library, but it would not be difficult to replace the server
with a different one (for instance, a Twisted server).  Notice that at
the moment the <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> server does not work with to Python 3.2 due to
changes to <tt class="docutils literal">asynchat</tt>. In time I fill fix this and other known
issue.  You should consider the server functionality still
experimental and subject to changes. Also, notice that since
<tt class="docutils literal">asynchat</tt>-based servers are asynchronous, any blocking command in
the interpreter should be run in a separated process or thread.  The
default port for the <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> server is 2199, and the command to signal
end-of-connection is EOF.  For instance, here is how you could manage
remote import on a database (say a SQLite db):</p>
<pre class="literal-block">
import plac
from importer2 import FakeImporter

def main(port=2199):
    main = FakeImporter('dsn')
    plac.Interpreter(main).start_server(port)
    
if __name__ == '__main__':
   plac.call(main)

</pre>
<p>You can connect to the server with <tt class="docutils literal">telnet</tt> on port 2199, as follows:</p>
<pre class="literal-block">
$ telnet localhost 2199
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
i&gt; import_file f1
i&gt; .list
&lt;ThreadedTask 1 [import_file f1] RUNNING&gt;
i&gt; .out
Imported 100 lines
Imported 200 lines
i&gt; EOF
Connection closed by foreign host.
</pre>
</div>
<div class="section" id="summary">
<h2><a class="toc-backref" href="#id50">Summary</a></h2>
<p>Once <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> claimed to be the easiest command-line arguments parser
in the world. Having read this document you may think that it is not
so easy after all. But it is a false impression. Actually the
rules are quite simple:</p>
<ol class="arabic simple">
<li>if you want to implement a command-line script, use <tt class="docutils literal">plac.call</tt>;</li>
<li>if you want to implement a command interpreter, use <tt class="docutils literal">plac.Interpreter</tt>:<ul>
<li>for an interactive interpreter, call the <tt class="docutils literal">.interact</tt> method;</li>
<li>for an batch interpreter, call the <tt class="docutils literal">.execute</tt> method;</li>
</ul>
</li>
<li>for testing call the <tt class="docutils literal">Interpreter.check</tt> method in the appropriate context
or use the <tt class="docutils literal">Interpreter.doctest</tt> feature;</li>
<li>if you need to go at a lower level, you may need to call the
<tt class="docutils literal">Interpreter.send</tt> method which returns a (finished) <tt class="docutils literal">Task</tt> object.</li>
<li>long running command can be executed in the background as threads or
processes: just declare them in the lists <tt class="docutils literal">thcommands</tt> and <tt class="docutils literal">mpcommands</tt>
respectively.</li>
<li>the <tt class="docutils literal">.start_server</tt> method starts an asynchronous server on the
given port number (default 2199)</li>
</ol>
<p>Moreover, remember that <tt class="docutils literal">plac_runner.py</tt> is your friend.</p>
</div>
<hr class="docutils" />
<div class="section" id="appendix-custom-annotation-objects">
<h2><a class="toc-backref" href="#id51">Appendix: custom annotation objects</a></h2>
<p>Internally <a class="reference external" href="http://pypi.python.org/pypi/plac">plac</a> uses an <tt class="docutils literal">Annotation</tt> class to convert the tuples
in the function signature into annotation objects, i.e. objects with
six attributes <tt class="docutils literal">help, kind, short, type, choices, metavar</tt>.</p>
<p>Advanced users can implement their own annotation objects.
For instance, here is an example of how you could implement annotations for
positional arguments:</p>
<pre class="literal-block">
# annotations.py
class Positional(object):
    def __init__(self, help='', type=None, choices=None, metavar=None):
        self.help = help
        self.kind = 'positional'
        self.abbrev = None
        self.type = type
        self.choices = choices
        self.metavar = metavar

</pre>
<p>You can use such annotations objects as follows:</p>
<pre class="literal-block">
# example11.py
import plac
from annotations import Positional

&#64;plac.annotations(
    i=Positional(&quot;This is an int&quot;, int),
    n=Positional(&quot;This is a float&quot;, float),
    rest=Positional(&quot;Other arguments&quot;))
def main(i, n, *rest):
    print(i, n, rest)

if __name__ == '__main__':
    import plac; plac.call(main)

</pre>
<p>Here is the usage message you get:</p>
<pre class="literal-block">
usage: example11.py [-h] i n [rest [rest ...]]

positional arguments:
  i           This is an int
  n           This is a float
  rest        Other arguments

optional arguments:
  -h, --help  show this help message and exit

</pre>
<p>You can go on and define <tt class="docutils literal">Option</tt> and <tt class="docutils literal">Flag</tt> classes, if you like.
Using custom annotation objects you could do advanced things like extracting the
annotations from a configuration file or from a database, but I expect such
use cases to be quite rare: the default mechanism should work
pretty well for most users.</p>
</div>
</div>
</div>
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